The University is expecting the yield rate to be higher this year, so I am guessing 41% or 42%. This means the yield rate has grown 7 percentage points (or a 21% increase overall) in 6 years, for the Classes of 2009 to 2015. Extrapolating forward, I expect the yield rate to grow another 7 percentage points or 20% overall (up to 48 or 49%) in the next six years.
This has already had some affect on other schools. The Yale Daily News noted last year that Yale has seen a slight but steady decline in yield for the past few years from 70% to 67%:
"(One college consultant) noted that while the slight decline in Yale's yield rate does not “cheapen” the Yale brand, there have been instances in the past where students have turned down Yale for schools such as the University of Chicago and Washington University in St. Louis, which both offer generous merit aid packages."
By the way, to see how UChicago's yield rate has changed even more over time, an article from ChicagoMag.com says that UChicago's "yield, or the proportion of students offered admission who ultimately enroll, was up 30 percent in 2009 compared with 1998." [Note: So the yield rate may have only been about 25% in 1998.]