Why might a Macbook Pro be freezing on resume from sleep?

From your kernel panic log:

Kernel loadable modules in backtrace (with dependencies):
     com.apple.nvidia.nv50hal(5.0.8)@0x3ce07000
        dependency: com.apple.iokit.IOPCIFamily(2.2)@0x35ea5000
        dependency: com.apple.NVDAResman(5.0.8)@0x3cbde000
     com.apple.NVDAResman(5.0.8)@0x3cbde000
        dependency: com.apple.iokit.IOPCIFamily(2.2)@0x35ea5000
        dependency: com.apple.iokit.IOGraphicsFamily(1.4.8)@0x3cadb000
        dependency: com.apple.iokit.IONDRVSupport(1.4.8)@0x3caf6000

That says that either Apple's Nvidia drivers or the Nvidia card itself is causing the computer to kernel panic so it may be a bad graphics card or simply bad drivers.

Now that Snow Leopard is out and you're planning to upgrade you can try an upgrade (which has newer drivers) and see if that fixes your issue. If the issue remains then it sounds like something is faulty with the graphics card and you'll want to contact Apple Tech Support for a repair under warranty (if you're still covered - 1 year with the computer, 3 years if you bought AppleCare)

Is it a GeForce 8600M? The warranty has been boosted to 3 years because of the card having a design fault - see Apple's technote.

Not a lot of experience with macs, but if this was a pc I would have to go down the route of background processes.

Virtualisation for me is the killer when it comes to resuming if they were ram intensive and did not pause.

If you can not fix it by uninstalling programs, personally I would take a full drive backup, restore to factory settings, and see if the problem has gone (hopefully you get this EVERY time, so you can easily see if working or not).

If this does not fix the problem, faulty hardware.

If this fixes the problem, I would reinstall the programs I use one by one until the problem occurs again. If it doesn't cross your fingers and continue using! if it does, you have a culprit and know what not to use!