This is actually an issue with PrettyFaces and Mojarra (the bug is actually in Mojarra/Glassfish, please vote for it!)
Thanks for the clarification Lincoln. Combined with the the previously mentioned bugs (one requiring PrettyFaces whether it's being used or not), it sounds like SeamFaces is unusable in Glassfish 3.1 unless we can do without ViewParams. Am I missing any workarounds?
For the benefit of anyone else coming across this thread: turns out the issue was with PrettyFaces after all and it will be fixed. See above links for more info.