After last nights update to Windows Marketplace for Mobile, we're getting lots of reports that Windows Mobile 6.0, 6.1 and unofficial WM6.5 builds are having problems accessing the store (myself included, using SSK 3.0).
One thing you can try to restore service is by using a .cab file that makes a change in the registry to "trick" the Marketplace.
It fixed my problem so it may be worth a shot. Simply download the .cab and install on your device, then re-launch Marketplace (soft-reset not needed). Leave us a comment on your success or fail!