Use Textmate (or any arbitrary application) with Lion's "Versions" feature?
Scouring the TextMate community and community plugins doesn't show that this feature is currently available in TextMate. This blog post states that they're "holding back with 'lionizing' TextMate till we feel confident we can fully drop backwards compatibility".

You're not alone though, the community seems to be very interested in this feature.

I don't have experience with this plugin, but the community seems to enjoy the ProjectPlus plugin.