I've seen numbers between 2dpi and 30dpi for Billboards. Depends on viewing distance, really. If your image looks good printed at 300dpi and held in your hand, that should be good enough and the rest, the printer should be able to do.
Consider a 10"x8" print at 300dpi. Print it at 10'x8', you've got 25dpi, but you'll be viewing it from much further away.
More information here.
Chris