Another fear -- that smokers will head en masse to Virginia. Luckily for DC, Virginia already discourages people from coming across the Potomac with its 2am last call (which translates to as early as 1:15am at many bars).
A much more likely scenario is that the smoking ban will be a benefit to DC businesses. DC already has the aforementioned 3am last call and the all-important Cool Factor. Now Virginia nonsmokers have another big incentive to head into DC. Montgomery, Prince George's, and Howard Counties in Maryland also have smoking bans.
For Virginia, it's one more area in which we're lagging behind. Virginia not only lacks a statewide smoking ban, it doesn't allow cities or counties to ban smoking on their own. (Because of the Dillon rule, localities only have the powers specifically granted to them by the state [thanks to Vivian for the correction]). You'd think a Democratic governor like Tim Kaine would support a smoking ban, but in fact he opposes it.
If you think Virginia should ban smoking in bars and restaurants (or at least allow communities to make their own choice), email Governor Tim Kaine, your state delegate, and your state senator now.
UPDATE: DCist agrees on the impact of the smoking ban, as do a vast majority of commenters.