oneforty is a great idea, well executed and Laura rocks! I wish them all the best. Also, if i were Twitter i would buy them now - great asset to have in the long run.
Agreed it's a natural extension of their little ad badges on the web interface.
Also think about the potential of using the "via Tweetdeck" etc. links to directly point to the app store. I understand that developers like them to point to their site, so maybe developers could choose. It would be interesting to compare the conversion rates of both, a twitter owned app store (or oneforty for that matter) and a developers homepage.