My interest has been in replacing changing my Nagios notifications from SMS messages to something free--all it takes is a major problem to start racking up a number of SMS charges. And unlike SMSes, you can tell Prowl to stop bugging you, if it gets that bad.
If you have Prowl, the following
curlcommand will send a free notification to your iPhone:
curl -d apikey=YOUR_PROWL_API_KEY -d application=curl-test -d event='testing from curl' -d description='this is a test from curl' https://prowl.weks.net/publicapi/add
It's that easy. Worked on the first try. Still left to consider is it I trust this as much as I trust AT&T's email-to-SMS gateway service, which has never been an issue for me.
It can also tie into Growl, and given the name I imagine this was its first intended use, so things happening on your Mac, like bittorrent downloads completing and adium messages, could also notify your phone.