The big initial success story for Seaside, DabbleDB, is shutting down, based on email I just received as a user:

You are receiving this letter because you have been a user of Dabble DB.

We are writing to notify you that we will be shutting down the Dabble DB service on May 18, 2011. After exploring other options including finding another home for Dabble DB or continuing to try to support it ourselves, we've come to this difficult decision. We deeply regret any inconvenience this may cause you.

Although this in no way invalidates Seaside as a dev platform (many many other sites are successfully using Seaside!), it's a bit sad that the "mothership" never found a way to really take off.