I guess you'd just want to look for products that support Time Machine. Otherwise, NASes tend to be OS neutral, except for installation "wizards" that you don't really need that run on Windows only and if you're interested in using bundled backup software.
NASes that support Time Machine:
Seagate BlackArmor family
Buffalo Link and TeraStations
Synology NASes
Tim;
Don't the new HP WHS based boxes also support Time Machine?
Larry
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