Microsoft's MacBU has announced that it is working to bring Solver to Excel 2008 as part of a future update.
Today, the best option for people who need Solver is to run Excel 2004 and Excel 2008 side-by-side, and switch to the 2004 environment for workbooks that require Solver.
However, we have been hearing loud and clear from our customers particularly in education that the side-by-side solution is suboptimal. For many people, Solver is a critical and necessary tool for coursework, and they want to work with Solver natively in the Excel 2008 environment. [...]
The Excel team is actively working to bring Solver to Excel 2008 as part of a future update. We have two distinct technical approaches and are exploring both. Once we have established which is the best option and have code that meets our quality bar we will announce a timeframe for availability here on Mojo.
Solver is an Excel add-in that allows linear programming in Excel, and was initially released for the Mac when Excel 2004 shipped but was dropped from Excel 2008 due to its dependency on Visual Basic for Applications (VBA) support.
Microsoft has previously announced that VBA support will be coming back to Mac Office in the next version.