Charging for SMS sounds great, but I think the problem would be that most cell phone users have an email address which turns into an SMS message -- allowing them to avoid being charged.  Something I could see them charging for would be more active monitoring (i.e. every 30 seconds instead of every few minutes), more knowledgeable monitoring (trying to log in with a specified user/pass), or monitoring response times from the server.

Yes, this is an excellent idea also. I really think there's a premium plan here.  

Given that they limitsthe # of sites they'll monitor at 100, the implication is they're monitoring quite a few sites. Even if they went $5/month for the premium service, I bet they'd make enough cash to cover hosting and end-of-month beer bashes!

http://mon.itor.us - another free Web20 website monitoring service uses Ajax desktop interface, live performance charts and may other cool features. Recommended!