Ok, that's understandable. My default thinking would be: offer e-mail only notifications for free users, but you made a good point.

Your target audience/market is obviously not the casual user/blogger type so it makes perfect sense.

Might I suggest a free account that is limited to email alerts? It probably wouldn't cost you much, and it wouldn't cut into your 'business class' business... but it'd be a nice way for small timers to get a taste of your service monitoring their personal stuff (and then maybe recommend it to the boss)

I'd guess that anyone who needs to receive text messages already knows about the email to sms gateways that their phone carriers provide.

But you don't really need this service for that. Nagios sends directly to those (just like any other email address).

Of course, the two-way SMS that lets you wake up the other guys if needed would break under this.

yeah, and many people disable those due to spam problems.