Billboard repro dpi is quite low and ranges from as low as 16 dpi up to 72 dpi but usually less than 30 dpi. There are lots of clients out there that think they need giant sized files to meet the demands of a billboard. I've recently been shooting a series of multi use images that have been used in a series of full size billboard placements (20' x 40'-50'). I've been providing 100 meg files from medium format scans, for all uses (magazine ads, posters, billboards, coffee mugs and toliet seat covers). With some good uprezzing and sharpening they have reproduced beautifully for the largest of uses...20 x 50 feet-ish.
I have friend who had an image purchased...shot with a canon 20d. The file was used for a highway billboard and he said it looked fine. It also depends on whether or not the image is going to occupy the entire dimension of the board.
The file size for a billboard is much less important because you have to factor in viewing distance and viewing time...5 to 8 seconds).
Are the billboards you are shooting for of the roadside variety? huge and not approachable, designed to be driven past at automobile speeds? no biggy
If they are approachable and smaller such as kiosk or bus shelter types...then you have to provide some pretty good res. 260 dpi on up at final dimensions... Those are the ones you have to worry about because people are standing around, looking at them and drawing mustaches and blacking out the teeth on them and such.