It looks like Twitter for iPhone, the screenshots are of it running in Mobile Safari on the iPhone, but surely it’s meant for Android tablets:
This web app allows us to provide a high-quality and consistent Twitter experience on high-end touchscreen devices – whether or not an official Twitter application is available. It was built from the ground up for smartphones and tablets […]
Because: (a) the only native Twitter client for Honeycomb right now is TweetComb, and apparently it’s not very good; and (b) who on iOS is using the web app and not the native app?
[UPDATE: Because of the recent Blogger outage, the original link is broken. I’ve updated the link to simply point to the Twitter blog homepage.]