One of our demo garden MGs brought a pepper seedling leaf in last week with small bumps on the bottom, near veins. It looked healthy otherwise and didn't have apparent active insects. Then the same thing happened to my pepper seedlings, so I decided to look it up.
The issue appears to be edema, which is an abiotic problem usually resulting from overwatering. Basically, the plant takes up water faster than it can use it. With a mild case like this, the simple solution is: don't water as much! If it persists I may also try adding calcium as a deficiency of that mineral can also be a factor.
Here is an article on edema from U. Conn. (doesn't mention peppers specifically, but I made the connection through a hot pepper growing forum, and the many photos similar to mine on the web also help with diagnosis). And on UMD's HGIC, results of edema at a later stage.
Seedlings are best watered deeply and then allowed to dry out on the surface before watering again. This also helps keep fungus gnats away. Make sure your pots have adequate drainage holes and water doesn't sit in the trays.
My pepper seedlings are beautiful and large! Too bad it'll be another month before I feel comfortable planting them outside...