I just lost my Blackberry 8700g from T-Mobile but the insurance check is coming in, so it's not that bad. The lucky thing was my girlfriend talked me into buying the insurance for it ($5.99 a month with T-Mobile) when I bought it so I'm getting a check from T-Mobile with $110 deductable. Still better than nothing.
But I have this question popped up in my mind this morning that I was wondering if the person who found it can use that phone or not because I claimed it lost with T-Mobile. I just don't know anything about blackberry so i think you guys might have the answer. I just hope not!

It shouldn't be a problem if you have password protected your BB. T-MO can probably just deactivate the SIM card and then put the BB ID on a ban list.

I didn't have the password on it (I know I should have, I'm feel dumb now!)

That sucks, sorry to hear.

even if u put the password on it, if the culprit inputs the password 10times, the device will be wiped clean, and it will be just a new.
there will be no passwords to stop the crimminal.

That just sucks so bad, I thought whenever you connect your blackberry to the phone network, it sends in some kinda ID. So if I claimed it lost, should T-Mobile be able to lock the device to connect to the network? (blacklist them or something like that?)

There is no such thing as a "Ban List" for GSM phones unfortunately =( They can get a new SIM and use your device =(

At the very least they would be able to use the device as an offline PDA.