1. It's probably dust (altho the shutterbug response was priceless).
2. It does no harm whatsoever. The mirror flips up and covers the focusing screen when the image is taken. So just let it be until you have the camera properly cleaned some time.
3. If it's really buggin' you (heh heh), get a bulb blower and give it a few puffs... That might dislodge the dust (or eggs) and blow them out of there. It's safer not to wipe, rub, disassemble or otherwise touch anything in there, unless you have a pretty good idea what you are doing. Camera repair techs get to practice on other peoples' cameras and have pretty much unlimited parts access at wholesale prices.