Sent Brad some config examples to make Squid do some reverse caching.  Squid has some pretty cool directives to do that stuff, but that's not what it was designed for.  He just sent me an email saying that he may just rewrite the userpic process to cache to disk intead of memory.  All the userpics are stored in the DB which can get overloaded at times.  By caching we could eliminate some of the DB load.  It's cached already, but only in memory which is limited.  By moving to a disk based cache, either Squid or something he writes it should speed things up.