We already have this. There's a beautifully-designed mobile site theme available for most if not all of the common phones (also see this question and the original feature request).
The developers really put a lot of love into this. It supports nearly everything that the full site does, all on a small screen and optimized for speed on slow connections. You can read all of the content, post questions/answers, vote on posts, view and leave comments, receive comment and other notifications, and even perform some moderation tasks like casting close/delete votes on questions (assuming, of course, that you have the requisite privileges).
If you have additional feedback for the mobile site, or you happen to have a device that has not been whitelisted to receive the mobile theme, suggest it by posting an answer to this question.
If that's not good enough for you (sheesh!) and you were expecting Stack Exchange to constantly ping your phone with text messages, then I'm afraid you might be out of luck. I suspect the overwhelming majority of users would never want that. If you absolutely need to get more jingle from Stack Overflow, consider creating and subscribing to a Twitter-as-RSS feed. (Also see: Can the SE sites tweet me when someone comments or answers my question?)