Yep, we've been planning this from the beginning but we've hit some hurdles along the way. Getting your Fuelly account associated with your Twitter account is a painful multi-step process, and we've been trying to come up with an easier way. And coming up with a good message format and how to handle errors (ignore the message if it's formatted wrong?, send a direct message back asking for a corrected version?, etc) is another issue we're thinking about.
This is in the works, we're just trying to find the most efficient and error-free way for it to happen.
Oh, this seems as good a place as any to mention that if you want to go the other direction and add your fuel-ups to twitter, I saw someone doing that the other day. They added their Fuelly fuel-up feed to TwitterFeed. Then it auto-posts every fuel-up to Twitter for them.