As, upgrading to the new and shiny OS X 10.7 Lion will for now mean that you can't use a third-party NAS server as the backup device for use with Apple's Time Machine software.
This is because Lion now uses a new version of Netatalk, which has different requirements from the previous version, making current third-party NAS servers incompatible with its filing protocol until their firmware is updated.
I tried the new OS with a few NAS servers in the last two days, and while Lion could still see the NAS server as the backup drive, the backup would fail to proceed, giving an error message that reads, "The network backup disk does not support the required AFP features."
Similar things would happen when restoring from backups made by Snow Leopard. You'd be able to enter Time Machine to view the backups but it would run into error messages when restoring.
Note that this only happens with backups that reside on the NAS servers, those that work with Snow Leopard and earlier versions of OS X. Lion's Time Machine works fine with backing up on an external hard drive or the Mac's secondary drive.
If you use a NAS server as a backup device for your Mac, you may want to check with the vendor about the supporting firmware update before upgrading to Lion.
If you use a server from Synology, the company has just released a beta of DiskStation Manager 3.2 that, among a long list of improvements and additional features, supports Lion very well, including Time Machine.