I use a ReadyNAS NV+ as my backup drive and bulk storage. Although the newer firmware directly supports TimeMachine, I've never been able to get that to work. (This probably has something to do with the fact that I was upgrading and downgrading my NV+ Firmware quite a bit to debug a separate issue).
However, I did find a great tool to create SparseBundles that you can use on a NAS (or any external disk).
BackMyFruitUp. First, it is a great name. Second, it is a simple and easy tool. The Tool I actually use is 'Create Volume Backup,' a subproject of BackMyFruitUp, which you can download from this page.
You hardly need instructions. Unzip it, run it, and type in the size you want for the sparsebundle. Then just copy it to your destination share and point Time Machine at it. Done.
Of course, I wouldn't need it now if my Time Machine SparseBundle hadn't become corrupted. Luckily I didn't need it. I also perform a seperate rsync backup on occasion to insure I have a 'basic' backup of my user directory as well.