TweetCaster HD for Honeycomb tablets is now fully integrated into the main TweetCaster distribution as of the 4.5 release.
When installed to a compatible tablet, the tablet will automatically get the HD version, while phones will get the appropriate phone version.
The HD version will no longer appear in the Market as a separate product, but the HD features are still available for compatible devices in the main TweetCaster installation. This is true of both free and Pro versions.