TextPayMe is an interesting idea. In short, the service allows you to send and receive money using SMS (text) messages on your mobile phone. That's a cool idea, but I wonder how useful it actually is.
If it were possible to say, pay a restaurant check with your mobile phone, then I would say definitely, yes, it's useful. But person to person payments aren't as common (at least not to me). The site suggests a few ways to use the service, one of which is to "settle your roommates' rent and utility bills on the spot," which I think is a pretty interesting idea. I know several people who have roommates and this might actually come in handy for them.
I would love it if this service got more and more popular and then extended to allow businesses to accept money in this manner (it seems to be planned, as evidenced by the mention of "merchant accounts" in the FAQs), so you could use your phone almost like a credit card. Especially useful would be the ability to pay for transit using your phone (like in Japan and other places). The MTA here in NYC have announced that they plan to switch to an Oyster-card like system in the future (proximity-based cards rather than the magnetic-strip cards we use now), and it would be great if you could use your phone as well. I think this idea is a starting point of the whole concept.
Of course, if any of those ideas were to come to fruition in the future, your mobile phone would become even more valuable, and losing it would become even more dangerous. Hopefully, we'll have better wireless security and such by then anyway.
In the mean time, TextPayMe gives you $5 free for signing up, and if you refer enough people, you can win an Xbox360. So check it out and sign up and get your free money (and help me win an Xbox). It's a cool idea with an interesting future.