If you are looking to use Seaside in Pharo 5, please go ahead and let us know if you run into any issues.
Seaside 3.1.5 and 3.2.0 (unreleased) load into Pharo 5 and all tests are green .
Also: I know the finalization of 3.2.0 is long overdue, so news about that is coming soon.
“Load release 3.1″
“Load pre-release 3.2″