I had the same problem with USB devices taking a horrendously long time to unmount in Windows 7 when I used the "Safely Remove Hardware" Task Tray icon.
Poster omg108, who solved this problem for him/herself while trying to fix a problem with Adobe Illustrator, ended up pointing me in the right direction. I was quite positive that I had a default printer installed, but pulled up the Devices and Printers window to look around anyway... and, while looking through my devices, I began to wonder if the CUPS (Linux) print server I'd installed late last year had anything to do with my USB unmounting problem; the CUPS server wasn't my default printer (a network printer was), but we all know that Microsoft products tend not to play terribly nicely with Linux. For the heck of it, I removed that device out of my Devices and Printers... and my USB ejection problem was gone.