Hmm - I'm pretty sure Snow Leopard should work just fine. If you search on Drobo's web site, they have excellent instructions (or the did at one time) for manually setting up Time Machine to backup over a network. 10.7 and higher require AFP (Apple File Protocol) for TimeMachine which initially Microsoft was going to support in WHS via an add-in, but that add-in never materialized and then MS cancelled WHS slamming the door shut on that solution
Thursby Software makes an AFP server for Windows Server but it's not exactly home server price friendly.