My absolute favorite feature is that all my log output shows up in Textmate's build panel with a clickable link, leading me directly to that place in code. All you need to do is make sure log output has a certain prefix (eg you need a custom NSLog replacement).
That sounds like an interesting tip for a blog post (hint, nudge, wink)!
I would second the opinion that it's probably smarter to keep Xcode for your project management tasks while you are learning. If you're working through tutorials or a book things will be much easier to follow along with, not to mention all of Apple's sample code.
I'm sure my setup is pretty simple but it basically amounts to me dropping the project folder on TextMate and using it for all editing/searching/navigating then switching to Xcode for build/run/debug type stuff. Getting familiar with the triggers for Cocoa and the general navigation aids is probably the most important thing to do. Command-T and command-shift-T are huge.
I too would be very interested to hear how others are using TextMate to eliminate trips to Xcode though.