epistable - 11:56 am on Nov 30, 2008 (gmt 0)

Less visiting time with more pages per visit, as you're seeing, may not be such a bad thing. It may mean that your visitors found what they were looking for faster (albeit with more navigation) than before. Have you done any work with a website optimization package? If you have something you really want your visitors to do (a goal), it would make sense to run a test to see which of the two layouts lead your visitors to accomplish that goal.