It works great for me, except for one thing: it always looks up symbols in the cl-user package, even when slime knows the source is in a different one. Other than that, it's good stuff!
I still have some reservations about using eldoc (mainly because the hook you use is very recent, and defadvise seems a little kludgy to me) when what it does here is I think can be recreated in around 40 lines (over which we have complete control). But I'll soon get over it :-)
One more thing in connection with this: is tracking the current package in the *inferior-lisp* buffer planned for slime?
Andras