Well Singletrack, let me take a few moments to provide a thorough answer to this question.
If you cut all the ingredients in half, and if you transfer the entire volume of your BK to the fermenter, then yes, you are correct: if you could also reduce your boil-off volume to half, then there would be no change in the calculated IBUs.
However, you know it may be very difficult to halve your boil-off volume, and that will result in a lower pre-boil gravity, since you will need a higher percentage of water pre-boil. And that lower gravity means higher hop utilization, which leads to more IBUs. How much more? Glad you asked. For the Belgian dubbel that I will be brewing this weekend, I calculated IBU=25.3 for the full recipe, and IBU=26.4 for the half recipe. Could you detect that 1 IBU difference? I know I could not.
But wait. Maybe you do NOT dump the entire volume of your BK into the fermenter. As the founding (and sole) member of BETH -- Brewers for the Ethical Treatment of Hops -- I insist on free range hops in the BK, so yeah, I leave some sludge in the BK. In this case, we have more to worry about than IBUs. For that dubbel mentioned previously, the full recipe had an OG of 1.072, and used a BK loss volume of 0.5 gal. (Don't judge me for my out-of-style "dubbel.") If I use the same 0.5 gal loss for the half recipe, the OG is only 1.067. This is a little different beer. But you should be able to cut the BK loss in half to solve this problem. Then, the IBUs will be very close again too.
So there you go. Just do what acidrain said to begin with! But cut the BK loss in half and use a smaller pot to reduce boil-off if possible.