Not strictly for commuting, but I'm not much of a hybrid fan either. How 'bout a Norco Bushpilot? I know zilch about these bikes, but it's in your price range and has discs. MSRP is $539 CDN
Or a roadie type commuter (Norco lists it as a performance commuter). MSRP is $709 CDN, but you might be able to pick it up for less if you get the '05 model.
And of course, you can always go fixed gear which means you provide most of the braking force w/ your legs and still have handbrakes for when you really need it. The Redline 925 goes for <$500 USD I think.
Best thing to do is to try out a bunch of bikes in your price range to see what you like.