Yes; the only thing in the Java world that has any kind of mainstream 
support and a similar basic philosophy to Seaside is Tapestry, but it 
suffers from both the Java environment and even more from the Java 
attitude to things like configuration: I get the impression that using 
it involves as much time modifying XML files as writing code...

If for some reason I was compelled to do web development on the JVM, I 
think the only way to go would be using something like SISC Scheme or 
the Rhino Javascript interpreter (both of which run on top of Java) so 
that you at least got a decently dynamic and high level language to 
work in.  Either of those could support a credible implementation of a 
Seaside-like framework, though still without the nice environment that 
Smalltalk provides.