This could be an effective way for the Dems to bring out a high black turnout in the midterms. Only the crazies and far righters believe this "Obama isn't a real citizen" nonsense so pushing that opinion on the Republican party as a whole could effectively mobilize the black vote and continue prying independents and moderates away from the GOP.
This is correct. If the Democrats minus Obama can publicize this as much as possible they have a LOT to gain. African-American turnout will be slightly, slightly, helped in the 2010 midterms if all of the crazies start coming out in force. This would at most translate into one seat.