If I buy a 3G unlocked Blackberry from Carrier "X" and use it with carrier "Y," would 3G work fine?

It depends. What carriers are you considering?

Sprint and Verizon devices are not interchangeable on those carriers, meaning you cannot put a Sprint device on Verizon and vice versa.

In the GSM world, ATT and TMO operate on different 3G frequencies that are not compatible.

And of course you can't put a GSM device on a CDMA network.

Hybrid devices like the 9530 are 3G on CDMA, but only EDGE on GSM.

Not sure what the future holds however.

And it helps us a lot if you at least show which device you have and your carrier in your profile. Some problems could be OS, device and/or carrier specific.

GSM device: Considering a BB 9700 from Rogers, to work with T-mobile service. So I guess the question is if Rogers and T-Mo are interchangeable/compatible.
It's interesting that 3G technology opens such a can of worms, since until now an unlocked phone of any GSM branding worked flawlessly with other GSM carriers (I am using the BB 8900 from Rogers, an eBay purchase, with T-Mo without any issues). 3G is now narrowing the possibilities...

No, they aren't.
Rogers uses the same 3G frequencies as AT&T in the U.S.
AT&T and T-mobile use different frequencies for their 3G networks.

Yikes.

"It's interesting that 3G technology opens such a can of worms, since until now an unlocked phone of any GSM branding worked flawlessly with other GSM carriers (I am using the BB 8900 from Rogers, an eBay purchase, with T-Mo without any issues). 3G is now narrowing the possibilities..."

you'd need to get a t-mobile 9700 to work with 3G on t-mobile.

Wirelessly posted (overwhelmingly stressed)

3G isn't narrowing the possibilities the network operators choice of which frequency they use for 3G is causing the limit!

Either way: eBay offers a limited unlocked Blackberry 3G phones now. The "unlocked" concept is limited, if you will.

Well, no.
It is unlocked, id expect it to work on any network that has the appropraite frequencies it can recognise.
I wouldn't expect an unlocked rogers bold to work on t-mobile's 3G.
But then again, I know it wont.

Right. But it won't work on the 3G technology of a different carrier (i.e. a Rogers 9700 won't work on the 3G of T-Mobile, etc). This is something that needs to be mentioned by an eBay seller. It's a new concept: "Unlocked Blackberry 9700 from Rogers that won't work on T-Mo's 3G." At least the buyer(s) needs to be aware of that. 3G phones right now carry a hefty price because they are 3G. So don't pay premium for a technology you can't get.

I agree, that sellers should make it clear.
But how many sellers know that information?
We know it, because its posted here.
I imagine alot of sellers are doing it because they can make alot of money from selling the devices, not because they know about blackberries.

Any buyer should research beforehand. Like you have done.
I know alot of people wont, and more than likely we'll have to explain to a good few people that they bought the wrong device for the network they want it on.

I think the buyer is the one that needs to be doing the research. If your only shopping criteria is "3G" then you haven't done enough homework. 3G could mean several different networks/frequencies. It's impractical to expect the seller to outline every single combination of device/carrier that won't work.

Its the same principal if your going to get a blackberry and want email.
Buyers need to research these things.
As we know, they don't.

I agree. That's why as a buyer I posted this thread. Having said that, the 3G technology is what makes phones now more expensive, so if you pay for a phone that in theory delivers 3G speed but when you get it it doesn't then the oops is on you.

Just like buying a Rogers 9700 and using it on T-Mobile and expecting UMA.
Oops.

Let the buyer beware.

It's possible! I paid $50 and got both My fav5 and UMA on a Rogers unlocked phone

So you got a Rogers 9700 to use UMA on T-Mobile?