I'd pre-order if I definitely wanted one, because then you don't need to hunt around or worry if you'll find your preferred model at a store.
T Mobile is offering a diff biz model in some ways, because they're offering unlimited data and no phone subsidy. Who knows whether they're stocking up conservatively and might mis-estimate demand? I think I remember reading that another carrier got left holding the bag on a lot of unsold iPhones because they mis-estimated demand and obligated themselves to Apple to selling way more than they actually ended up selling, which was going to cost them. Maybe T Mobile will play it safer, esp because of their diff biz model.
Besides, T Mobile seems to be goofing up its pre-orders, so maybe they don't actually know what the heck they're doing:
Four Days Until The T-Mobile iPhone Goes On Sale?How Are Those Pre-orders Going? ? Consumerist