Please help me understand how a service provider controls the feature set on an unlocked GSM phone.

I understand that if I buy a GSM phone from one of the GSM service providers (AT&T, Cingular, T-mobile) that the phone will be "branded" by the vendor (start-up splash screen I imagine) and that that vendor's "special" features will be enabled. I also understand that some vendors may disable some functionality (e.g, on the upcoming 650, the ability to use BT as a modem).

Here's the question:

If I buy an unlocked GSM 650 from palmOne, and use it with one of the GSM service providers, I assume I'm getting palmOne's "standard" software load which, I also assume, would have all functionality enabled. Is it then correct that all that functionality would be available to me regardless of which GSM service provider I signed on with - and even if that service provider had crippled that functionality on its branded phones?

Similarly, in going with an unlocked phone direct from palmOne, I assume (a really dangerous word) I'll be missing some of the special features or software load that the individual GSM service providers bundle with their phones. Is this a major issue or disadvangage?