From what I know, Booth received 4200 applicantions for class 2008. The acceptance rate is 22% and yield 60%. With these data in hand we can do some number crunching.
Number of offers extended= 4200 * 22% = 924. Just as a check , class size = 924 * 60% = 554, coincides with Chicago's official class size.
Round 3 typically admits least applicants, let's assume distribution of offers for the 3 rounds are 2:2:1. then round 1 we shall see 2/5*920=370 offers. Given this year's applicant pool is very likely stronger than last year's, R1 offers might reach 400.
So that gives us a range of 37% (worst case scenario based on our assumptions: 370/1000) to 44% (best case scenario: 400/900). This roughly matches what I was told at the info session: they admit about 40% of those who were interviewed. But of course things change every year. By using last year's data, we are merely looking into the rear view mirror here.