OP: I assume you're talking about the average GPA of accepted students (or incoming freshman...slightly different, but same idea), and not the average GPA of the college. You may want to clarify.
Most colleges will use their own formulas to recalculate GPA for incoming students. Some schools won't count "non-academic" courses (shop, home ec, some theology classes, PE, etc.), some won't weight, some will weight all H/AP/IB by a full point, some will weight H by 0.1 point and AP/IB by 0.25, some will weight +/- differently...it varies.
I believe that average incoming GPA's are typically reported using whatever weighting system the college employees. As a random example, Berkeley's 2007 freshman profile reports an average unweighted GPA of 3.82 but an average weighted GPA of 4.25, and both of these are "UC GPA's," meaning that they only count academic courses, and the number of weighted classes is capped. I've personally seen the weighted GPA more often, but even that number only makes sense in a context. The school's recalculation might make your high school GPA *as you know it* go up or
So, in general, this is probably the product of high-achieving applicants, widespread HS grade inflation, and weighting. Remember that you'll be evaluated in a context, so don't stress too much if you have a 3.5 but are #1 in your class, or go to a school that doesn't offer AP courses...it'll be taken into consideration.
ETA: For anyone interested, this link regularly pops up in these discussions: http://www.gradeinflation.com/