Here's from the forum... thought I'd find it:
Beersmith (and pretty much all homebrewing software) gets this wrong pretty dramatically. The formulas used by software are based on the old, mistaken belief that hop utilization is affected by wort gravity (pretty much all home brewing texts convey that myth too, though I'd expect it to be corrected in forthcoming additions of How to Brew and others).
Hop utilization is independent of wort gravity, and the impact (if any) of late extract addition on IBUs is much, much smaller than what software will calculate.
The most recent test on this was Basic Brewing Radio's experiment where they brewed the same recipe (same hop schedule) as a full boil, partial boil, and partial boil with late extract additions, then measured the IBUs of the 3 beers in the lab. Hop utilization was essentially identical (the three came out with nearly the same IBUs).
March 4, 2010 - BYO-BBR Experiment III:http://www.basicbrewing.com/index.php?page=radio
You can listen to John Palmer's "What is an IBU, Really?" from 20 March 2008 where he discusses the issue in some depth (including apologizing for getting this wrong in the most recent edition of How to Brew):http://www.basicbrewing.com/index.ph...ing-radio-2008
Now, there are some effects that often correlate with wort gravity that can impact final utilization--e.g. isomerized alpha acids can adsorb to break material. Those are much smaller effects than what brewing software calculates, though, and are pretty minimal in extract brewing (see the test above).
More discussion here:http://www.homebrewtalk.com/f128/est...te-art-109681/http://www.homebrewtalk.com/f14/hop-utilization-178668/