Adium 1.4b9 is Available for Snow Leopard
It's not exactly clear what the problems might be with Adium 1.3.x on Snow Leopard, but in case it's really bad, there is a 1.4 beta that's available for Snow Leopard. It too, has issues with the font rendering, it seems, but maybe that's not so bad, given the problems 1.3.x might have. Too early to tell, but if I upgrade to Snow Leopard, and Adium 1.3.x is unusable, I have an option.
UPDATE: this compatibility list says that Adium 1.3.6 is OK for Snow Leopard - good to know.