There's a hybrid solution to this problem that allows you to retain the use of text messaging UI on your phone, use your data plan to transmit the texts, and view/store/transmit/receive them online at a "real" computer. However, it requires an additional phone number be disseminated to your cohorts and you sign an additional component of your life over to teh Google.

http://www.google.com/voice (www.google.com)

I'm not sure if it currently requires an invite or not. Ultimately, though, it appears to meet all the previously discussed criteria, if access to the service itself can be obtained. I've had an account since their Grand Central days, and other than a bit of lag with the iPhone app, it seems to behave quite well and function as advertised.