From Solver.com
All Solving methods are limited to 200 decision variables; the GRG Nonlinear and Evolutionary Solving methods are also limited to 100 constraints in addition to lower and upper bounds on the variables.
As far as I know you got 3 Solver models in Excel2010, the LP (linear), the GRG Nonlinear and the Evolutionary method so have you tested them all and with the same result?

If the model still works in Excel 2007 it is possible to keep the "old" 2007 Excel when upgrading to Excel 2010. You must choose "Customize" when you upgrade and then select what previous office components you wish to keep.

Alf