BG,
will any memory card work for the 8100 series? For example, a Sony Memory Stick Pro Duo?

According the specs I've read, it will use a MicroSD card. Sony memory cards will not fit. It is MicroSD.

MicroSD/Transflash only...

I like the fact that the card will be a tiny MicroSD, not something that sticks out like a standard SD probably would have. My Treo's card was always getting bumped and ejecting, a mild pain in the butt, but a pain none the less.

and just in case people are wondering. 2GB is the largest microsd out right now at about $99.

4GB isn't far off but no telling if the BB will have its own limitation of how large of a card it can see.

Would this be the same type card used by the Motorola SLVR?

Yes and it looks like the card slot will be under the battery. I hope it boots fast.