If you use a Samsung Focus WP7 device, then you have a “Consumer Accessible” MicroSD card slot into which you can plug a card of your chosing.
Unfortunately, your favorite brand may not be compatible with WP7 devices, but you wouldn’t know that because Microsoft has decided not to publish a list of Compatible cards at this time.  Since most of the WP7 devices do not have a Consumer Accessible slot, it was not deemed a high priority for apparently good reason.