CLLIB:OPEN-URL seems to be chewing up memory. Evaling either (close (cllib:open-url (cllib:url "http://yahoo.com"))) or (cllib:with-open-url (stream "http://yahoo.com") ())chews up ~ 200-400K.
I think the problem is that background processes aren't being cleaned up. Check (mp:list-all-processes) on Lispworks, sys:*all-processes* on Allegro - you'll find a number of sleeping "Timeout Monitor" processes waiting around unused.
I could not replicate the problem using (port:with-timeout (5) (princ "in process")). The timeout processes may have a stream open or something, and are failing to get cleaned up? So, I'm assigning this to CLLIB.