I've noticed this a few times: I'll be using SLIME and suddenly space no longer causes the function arglists to show up in the minibuffer. SPC is still bound to slime-space but it doesn't work in either Lisp source code buffers or REPL buffers. I certainly didn't do anything on purpose to turn it off. I just observed it in Allegro though since I switch impls a lot I'm not sure if that's the only place I've seen it. I'll try to look into it if I get a chance but I thought I post here in case anyone else knows anything about it.
-Peter