Why not ... everyone else is in the race. Actcually, my father and I discussed this possibility just last night. From US News:

With many Democrats unenthusiastic about the nine presidential candidates, two more are sizing up the race and could join the bid to take on President Bush by September. A top Democratic official says that Delaware Sen. Joe Biden is at least 50-50 on joining "and some days is 70-30." A family member says it's closer to "80 percent" a go.