It is currently not possible to do an offline install of JBoss Tools 3.1.0GA from the p2 repo zip file.
It seems the provided site master archive is incomplete (http://downloads.sourceforge.net/jboss/JBossTools-Update-3.1.0.v201003050540R-H56-GA.zip)
When attempting to install offline, the Smooks UI plugin fails to find a required dependency, generating the log attached below.
This behavior can be reproduced by following the official installation instructions, but modifying step 8 by unchecking "Contact all update sites during install".
http://www.jboss.org/tools/download/installation/update_3_1.html#fromzip
The only alternative is to install with a connection to the internet, which is not always possible.

Image of the problem: http://imgur.com/DqJhF.png
Log:
!SESSION 2010-04-22 19:35:41.471 -----------------------------------------------
eclipse.buildId=unknown
java.version=1.6.0_20
java.vendor=Sun Microsystems Inc.
BootLoader constants: OS=win32, ARCH=x86, WS=win32, NL=en_GB
Framework arguments: -product org.eclipse.epp.package.jee.product
Command-line arguments: -os win32 -ws win32 -arch x86 -product org.eclipse.epp.package.jee.product

!ENTRY org.eclipse.equinox.p2.ui 4 0 2010-04-22 19:36:31.018
!MESSAGE Operation details
!SUBENTRY 1 org.eclipse.equinox.p2.director 4 1 2010-04-22 19:36:31.018
!MESSAGE Cannot complete the install because one or more required items could not be found.
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.019
!MESSAGE Software being installed: Smooks Tools 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.feature.feature.group 1.1.0.v201003050540R-H56-GA)
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.019
!MESSAGE Missing requirement: Smooks Tools UI Plug-in 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.ui 1.1.0.v201003050540R-H56-GA) requires 'bundle

org.eclipse.zest.core 0.0.0' but it could not be found
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 1 2010-04-22 19:36:31.019
!MESSAGE Cannot satisfy dependency:
!SUBENTRY 3 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.019
!MESSAGE From: Smooks Tools 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.feature.feature.group 1.1.0.v201003050540R-H56-GA)
!SUBENTRY 3 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.019
!MESSAGE To: org.jboss.tools.smooks.ui [1.1.0.v201003050540R-H56-GA]

!ENTRY org.eclipse.equinox.p2.ui 4 0 2010-04-22 19:36:31.020
!MESSAGE Operation details
!SUBENTRY 1 org.eclipse.equinox.p2.director 4 1 2010-04-22 19:36:31.020
!MESSAGE Cannot complete the install because one or more required items could not be found.
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.020
!MESSAGE Software being installed: Smooks Tools 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.feature.feature.group 1.1.0.v201003050540R-H56-GA)
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.020
!MESSAGE Missing requirement: Smooks Tools UI Plug-in 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.ui 1.1.0.v201003050540R-H56-GA) requires 'bundle

org.eclipse.zest.core 0.0.0' but it could not be found
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 1 2010-04-22 19:36:31.020
!MESSAGE Cannot satisfy dependency:
!SUBENTRY 3 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.020
!MESSAGE From: Smooks Tools 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.feature.feature.group 1.1.0.v201003050540R-H56-GA)
!SUBENTRY 3 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.020
!MESSAGE To: org.jboss.tools.smooks.ui [1.1.0.v201003050540R-H56-GA]

!ENTRY org.eclipse.equinox.p2.ui 4 0 2010-04-22 19:36:31.024
!MESSAGE Operation details
!SUBENTRY 1 org.eclipse.equinox.p2.director 4 1 2010-04-22 19:36:31.024
!MESSAGE Cannot complete the install because one or more required items could not be found.
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.024
!MESSAGE Software being installed: Smooks Tools 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.feature.feature.group 1.1.0.v201003050540R-H56-GA)
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.024
!MESSAGE Missing requirement: Smooks Tools UI Plug-in 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.ui 1.1.0.v201003050540R-H56-GA) requires 'bundle

org.eclipse.zest.core 0.0.0' but it could not be found
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 1 2010-04-22 19:36:31.024
!MESSAGE Cannot satisfy dependency:
!SUBENTRY 3 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.024
!MESSAGE From: Smooks Tools 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.feature.feature.group 1.1.0.v201003050540R-H56-GA)
!SUBENTRY 3 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.024
!MESSAGE To: org.jboss.tools.smooks.ui [1.1.0.v201003050540R-H56-GA]

!ENTRY org.eclipse.equinox.p2.ui 4 0 2010-04-22 19:36:31.065
!MESSAGE Operation details
!SUBENTRY 1 org.eclipse.equinox.p2.director 4 1 2010-04-22 19:36:31.065
!MESSAGE Cannot complete the install because one or more required items could not be found.
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.065
!MESSAGE Software being installed: Smooks Tools 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.feature.feature.group 1.1.0.v201003050540R-H56-GA)
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.065
!MESSAGE Missing requirement: Smooks Tools UI Plug-in 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.ui 1.1.0.v201003050540R-H56-GA) requires 'bundle

org.eclipse.zest.core 0.0.0' but it could not be found
!SUBENTRY 2 org.eclipse.equinox.p2.director 4 1 2010-04-22 19:36:31.065
!MESSAGE Cannot satisfy dependency:
!SUBENTRY 3 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.065
!MESSAGE From: Smooks Tools 1.1.0.v201003050540R-H56-GA (org.jboss.tools.smooks.feature.feature.group 1.1.0.v201003050540R-H56-GA)
!SUBENTRY 3 org.eclipse.equinox.p2.director 4 0 2010-04-22 19:36:31.065
!MESSAGE To: org.jboss.tools.smooks.ui [1.1.0.v201003050540R-H56-GA]