It costs money. You have to have servers with Windows licenses, which we don't have. Also, I like the way Linux organises its "disk protocol stack" - i.e. as a set of interchangeable protocols that you can kind of slot together as you wish. If I understand this right, DFSR replicates folders. DRBD replicates entire file systems, regardless of file system. It acts as a Linux block device, interchangeable with other block devices. I'm thinking of adding an encryption layer on top of DRBD, which will (hopefully) be a case of simply telling Linux to create another block device from the DRBD device and encrypt it.
why did you not use Server 2003 R2 and mirror using DFSR and Virtual Server?