You really want an SMS alert every time someone tweets, but you don't actually want to follow that person on Twitter? No problem. Twitter's Fast Follow feature gives you just that - even if you don't have a Twitter account.
All you need to do is text ‘follow Username' to 40404 and Twitter will take care of the rest - and of course you can stop the SMS alerts at any point. Do note that the feature is currently only available in the US. [Twitter]