Via Glenn Vandenburg, I came across this post from Reginald Braithwaite-Lee. It's all good, but there's a really, really good summary in there, where he explains why Rails is hot and Seaside isn't (yet):

Speaking of Rails, I'm going to conclude with my take on one reason why Rails is taking off and Seaside is not. Rails allows programmers to express the idioms they already know (relational databases, web-backed MVC, stateless event handling plus a global session store) in fewer bits.

Seaside provides a whole new idiom, continuations, that IMO is more powerful. I think you end up with an even higher signal-to-noise ratio with a Seaside app than with a Rails app. Why? Because continuations afford you a much higher degree of controller reuse.

Now, here's the catch: if you try to imagine your current application running on both Rails and on Seaside, you probably won't see much difference between the two (although they'll both be an order of magnitude better than ASP.NET). They will look the same because you designed your application with idioms that both Rails and Seaside support.

I think that captures it perfectly. What we (in the Smalltalk community) need to do is show Seaside (or something else in Smalltalk) solving a common problem in a dramatically easier fashion - as Rails has.