If you're a G1 owner now, you know that you have full access to the Marketplace without any overt interference from T-Mobile. T-Mobile has no policy that we only use their app store.
Now, to be fair, Verizon isn't excluding what they consider to be 3rd party app stores like Marketplace. It's just that they won't endorse it and won't help you access them. You'll have to do it yourself.
Honestly, reliable network or not, I hesitate going to a network where it has open stated it will be the gatekeeper of wireless Internet access. It seems like all their talk about an open network is just that. Talk.