Hi Joe, I just discovered this today while writing up some end user documentation. I'm guessing a change on the Twitter side may have broken things recently.
Tweet This generates a URL like:
http://twitter.com/home?status=What%27s+Up%2c+Doc%3f+http%3a%2f%2fwwwdev.escondido.org%2fwhats-up-doc.aspx
This results in nothing being defaulted into the "What's Happening" box when I click it.
I played around with a few scenarios and had some minor success, but I couldn't come up with a URL parameter combination that would reliably carry through to the What's Happening box, whether you are logged in or out of Twitter.
We're going to use AddThis for now, although I'm not real happy with how they add their own advertising to tweets (manually removable, of course). On the plus side, it's nice how they automatically shorten URLs for you.
Jamie