First, the good stuff. A new company called Twe2 will automatically monitor your incoming @replies and direct messages, then send them via text message to your phone for no cost. Up to 100 messages can be sent to you per hour, and you can choose whether to just receive @replies, just DMs, or both. Plus you can tell Twe2 which days and times you'd like to receive messages.
Now the less awesome stuff. Twe2 is powered by a service called Wadja, and to support the free service, your tweets come with an advertisement at the end of the text message. It's contextual, so keywords are picked from your incoming messages and relevant ads are tagged on at the end. This is exactly how Google's email service Gmail chooses which ads to display alongside your emails.
The service also doesn't handle regular tweets from friends -- ie, those that aren't replies or direct messages -- so it's not going to replace any all-encompassing future deals Twitter strikes with network operators, such as thewith O2. You'll also need to give Twe2 your Twitter username and password.
We tried out the service and received a direct message via text message 14 minutes after it was originally posted on Twitter. The small text ad simply advertised Wadja's free Web-based SMS service. In fact, all of our received @replies did.
Privacy concerns aside, Twe2 is a smart idea if you only care about reading responses to your own tweets, or your private incoming messages. But for us at least, it'll only be an interim service until Twitter announces an official partnership with network operators in the UK, restoring SMS alerts for all incoming tweets.