Yes, they can be used on T-Mobile once unlocked, but be warned: you don't get full functionality. Getting the Blackberry built-in browser and MMS may not be possible because T-Mobile doesn't have the necessary service books to sent to your Bold.
Unlocking requires you to purchase the unique unlock code from a vendor who supplies them. Check out Horizon Wireless
for the best service. Or you can simply buy a Bold from them, already unlocked. I'm using one from Horizon now on AT&T. It's an unlocked Rogers unit and functions perfectly!