[countdrachma]

I asked the exact same thing in a previous thread (not sure how to reference it here). The answer in short (based on the response I got back from Lizard / Omni) is that it didn't make sense to update the Omnifocus UI to support fullscreen mode when there are "other plans afoot" (paraphrased).

It would appear that this fullscreen support may be enacted upon in the impending OF v2 release; reading between the lines. You'll notice that the OO and OG updates were brought on in time for the Lion release as these offer the full feature support of Lion (auto-save, version and full screen). This makes sense because these are document-based applications. Omnifocus is not. Because it's a database / sync model, none of these features (except for fullscreen mode) are actually relevant to Omnifocus. Hence, there was (I'm only surmising here) no real push for Lion features for Omnifocus as a priority and interim addition given they are currently working on OF v2 anyway.

Cheers