Can't open WMV files in Quicktime since Snow Lep upgrade.....

The file codecs may not be compatible with QT X. Did you install QT 7 from the optional installs package on the disk? Alternatively, it appears that there is a flip4mac BETA for Snow Leopard http://dynamic.telestream.net/downloads/download-flip4macwmv-beta.asp

I had this problem. Double clicking the WMV file produced the message you had. However, if I open by dragging the WMV file onto the QuickTime X icon in the dock it did indeed open.

I then looked at the WMV's Get Info window and found that it was set to open in WMV Player by default. Obviously I do not have WMV Player installed. The drop down menu on this setting did have QT Player as the other option. When I changed the option to QT Player as default the "Change All" button became active. Previously it had been greyed out. Things have been OK since, but I have not tried it on any new WMV files.