I experienced the same problem. Here's the solution I figured out...
# Close Windows Media Player
# Unplug your Windows Mobile Device
# Navigate to C:\Users\[your username]\AppData\Local\Microsoft\Media Player
# Back up the contents of that folder
# Delete the contents of that folder
# Start Windows Media Player...it will scan your media library
# Plug your mobile device in, and Media Player will recognize it and any
storage cards installed in it.
You can see further details of this on my blog...