If you've installed a custom ROM and have trouble accessing Windows Marketplace, try this:
Use a registry editor and navigate to \HKEY_LOCAL_MACHINE\System\Versions
and change the value for registry key Aku to .5.3.00
I found this on another site a few months ago, and now I can't find it again. Wife got replacement phone, and I finally figured out which key I had changed to make it work.
Obviously, if your Windows Marketplace works OK, you don't need to make this change.