Hoster
The advantage of the TT mount is that it has a GPS enhancer.
I have finally succumbed to buying one after using my iPhone 3GS TT app for over a year, as I do get frustrated when the app loses position. Presumably, if you have an iPhone 4, you don't have the same problem?
There are dozens of non-TT holsters on the market. Rather than recommending one, I would suggest you get one which allows the TT to breathe - it's working awfully hard and those where the rear is totally encased cause it to get hotter and hotter. You can either select a claw type, which grips the edges, or one similar to the TT where there's lots of space around for air to circulate.
Also, decide between an air vent mount (which I found provided worse GPS coverage) and a windscreen mount. I currently have mine mounted in landscape mode above the rear-view mirror, and apart from the trailing lead, is very neat and easy to operate. If you're mounting near the front of the windscreen, you may have trouble reaching it and driving a car at the same time.
If you're getting a car charger with it, get one which will provide at least 1 amp - those giving 0.5 amp simply won't do the job.