Right now, alternate JPA providers (such as TopLink) are unusable in Seam. This is because, prior to the render response phase, Seam attempts to put the touched persistence contexts into manual flush mode. This trickles down to a call to setManualFlushMode() on the component returned by PersistenceProvider.instance(). The generic implementation (PersistenceProvider) throws an exception when this method is called.
Throwing an exception here is overkill and hostile. I propose a log warning message, which would allow the developer to tune the log settings to hush it, or if they really don't like the log message, they can provide their own implementation. But at least the damn thing will be usable.