It's pretty simple - they haven't used 0Auth yet (which is Twitter's authorization system, but they expect to by tomorrow i.e. 19th August 2009) but you can safely sign in with your credentials.
Here's how it works briefly: you send an sms with your registration details to a specific number (they have network specific numbers, but you're free to choose which number you pick) and once your registration is confirmed, you'll get all tweets from your Home screen.
Don't want to receive updates from a specific user? Send "off
You can even DM your fellow friends, and receive them too.
I know this service will help reduce the SMS email costs that I have to pay Telenor - since it's a perfectly good alternative to receiving news on your cellphone for free.
So far, all networks are supported, but the code for Telenor subscribers isn't up yet (you can use another telco's number instead - since it won't make a difference).
The website isn't much - it only gives you instructions. I guess they're trying to get the SMS service up and running first, then possibly make a front-end later on. But it definitely looks promising (and I went WOOHOO! on Twitter today). They're also on Twitter too!