Default Bad IMEI N900T Question

I've searched around for a definitive answer on this, but haven't found one.

I see a lot of N900T's advertised as unlocked with a bad IMEI. Does this mean that since it's unlocked I can use the phone with AT&T in the USA, just not with TMo? Does it depend on the specific N900T at hand or is there a uniform rule?

Thankful for any and all help.