I don't know what it's worth, but Brewing Quality Beers by Byron Burch outlines the following simple formula:
IBU = m60 * a60 * 4.0 + m30 * a30 * 1.4 + m15 * a15 * 0.7
where m60 ounces of hops of a60 alpha acid rating are added at 60 minutes, m30 ounces of hops of a30 alpha acid rating are added at 30 minutes, and m15 ounces of hops of a15 alpha acid rating are added at 15 minutes. The thing that Burch points out from this formula is that hops boiled for 60 minutes provide more than twice the bittering effect than the same amount of hops added at 30 minutes.
I recently started a Paulaner hefeweizen clone with 1.0 oz. of Vanguard hops (4.4% AA) that were added at 60 minutes with nothing afterwards, so the formula for me simplifies to:
IBU = 1.0 * 4.4 * 4.0 = 17.6
Note: this formula assumes a 5 gallon batch. Burch writes that if you need a different quantity, divide the final result by (dv/5.0), where dv is the desired volume of your batch. So, if you wanted to do an 8 gallon batch, you'd divide your IBU result by 1.6 to get the true IBU estimate for 8 gallons.
Hope that helps.