Apple is currently paying a lot of money for an ad that pitches Apple Macintosh as an alternative to Vista. That's bizarre, since 10% of all new Vista licenses are sold to Macintosh owners. Considering Apple's small overall PC market share, Mac owners are great Vista customers, and Vista runs great on Macs. Apple has shaped up to be a fine hardware OEM for Vista. I guess if they've decided that the weird "us vs. them" thing sells product, more power to 'em.
While many web developers run Vista and Expression tools on their Macs, it is possible to do some of your Silverlight development work in OS X. Check out Don Burnett's post explaining how to use Adobe Fireworks CS3 on OS X to create designs for Silverlight. This isn't just a proof-of-concept; I've met designers who haven't yet switched to Vista on their Macs, who do this every day.