interesting... del.icio.us supports a notification system like that. If you tag a URL as "for:jm", it'll become visible in my inbox, and therefore in my inbox feed (which I'm subscribed to).
This is a reasonably common feature in dynamic languages like Perl, Python and Ruby.
some of the things it allows are mindblowing; it gets even better when variable scoping is modified to allow "closures". You know the java pattern whereby you need to define an interface to receive a callback, then callers need to define instances of that interface etc. etc.? That becomes a single line of code in perl ;) it's truly beautiful in terms of simplification and readability.