jackonaplane - 8:59 am on Apr 30, 2012 (gmt 0)
I saw a similar situation to what you described after one of last year's Panda updates. Although your total traffic levels are the same, I would expect that it is indeed different somehow. Possibly more ad blind, possibly less likely to stay on your site for more than a few seconds, possibly even a very different demographic.
Have the referring keywords/sources of your traffic changed significantly? If so, then that would be an indicator that you're now getting a different set of visitors.