Resizing partition in Windows 7: how long does it take?
Solution 1:
Okay, found the answer. Let's leave it here for future reference:
http://incoherenttruth.wordpress.com/2011/01/05/things-you-didn%E2%80%99t-know-about-volume-shrink-in-windows-7/
The article reveals that the disk management utility in Windows 7 calls the OS' defragment tool. If you have the Disk Defragmenter GUI open, you can follow the progress of defragmenting.
You can also cancel the defrag process, which will cause the volume shrink to fail gracefully, with a message saying the shrink operation was cancelled.