I created a small web application with resteasy 2.3.4 Final, and I deployed it to Tomcat 7.0.30. I got the following error message when tomcat starts:

...
INFO: JSF1048: PostConstruct/PreDestroy annotations present. ManagedBeans methods marked with these annotations will have said annotations processed.
Sep 11, 2012 9:28:08 PM org.apache.catalina.core.StandardContext filterStart
SEVERE: Exception starting filter org.jboss.resteasy.plugins.server.servlet.Filter30Dispatcher
java.lang.NoClassDefFoundError: javax/enterprise/context/spi/Contextual
at java.lang.Class.getDeclaredConstructors0(Native Method)
at java.lang.Class.privateGetDeclaredConstructors(Class.java:2404)
...

My web.xml is as follows:

<context-param>
<param-name>resteasy.servlet.mapping.prefix</param-name>
<param-value>/services</param-value>
</context-param>

<context-param>
<param-name>resteasy.scan.resources</param-name>
<param-value>true</param-value>
</context-param>

<servlet>
<servlet-name>Resteasy</servlet-name>
<servlet-class>org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher</servlet-class>
</servlet>

<servlet-mapping>
<servlet-name>Resteasy</servlet-name>
<url-pattern>/services/*</url-pattern>
</servlet-mapping>