I've been using Dell's excellent ExtPart.exe for resizing Windows 2003 drives live.
But I found recently this utility does not work on Windows 2008 64 bit.
So the updated procedure for windows 2008 is:
1) use vCenter to grow the Windows 2008 drive
2) run Windows 2008 Server Manager->Storage->Disk Management->Action->Rescan Disks to have the added space from step one recognized
3) run diskpart.exe from the command line, list volume, select volume X, extend, list volume to verify space is added (live)!