Buying an Unlocked Android

[internetuser]

I'm thinking about buying a high end T-Mobile or AT&T branded phone from one of the well known online vendors (e.g. Negri, Mobilecity, GSM Nation) but am wondering if these phones are permanently unlocked. Does anyone know if they are unlocked by IMEI code or by software? Will these unlocked phones will survive software updates? And does anyone know if the phones that are due to update to Ice Cream Sandwich will still get it if I buy the unlocked version off of one of these vendors?

Thanks

[CGY Guy]

Why not just buy one from T-Mob, or ATT and unlock it yourself for $20 or so? Plus, you'll still be guaranteed a warranty with the carrier. It's a no brainer.

[lmcjipo]

You could also just get one of the Nexus phones (Nexus-One, Nexus-S, Galaxy Nexus) which are by default factory unlocked.

[internetuser]

cgyguy,
I don't want a contract. If I get one from the carrier without a contract, they charge me retail anyways.