I'm not sure Linux will ever pose a serious threat to the "home-user" market - even developers I know tend to agree that Linux has never taken user-interaction seriously. It might be prettier these days but that isn't the concern, it's genuine "getting things working" and "using this day by day" that is the problem. Most Linux users live by the terminal but home users moved to the GUI a long time ago and aren't going back.
OS X success is not about "pretty", not in a fonts, colors, and visual design kind of way. I'd warrant Vista is much "prettier" in that regard.
What is "pretty" about OS X is the capital D design of the overall user experience. Achieving this requires deep integration of goals between developers, ui experts, product managers, and yes, visual designers.
Otherwise it will only be like, shall we say, lipstick on a pig (a very tasty and fine pig, or good pet pig if you aren't into the swine flesh, at that).
When you add Linux snapping at their heels with a free desktop, I think they've got real problems. Especially when Shuttleworth is calling on Linux developers to make something prettier than OSX. They won't necessarily succeed, (although I hope we do), but simply playing in the space greatly complicates Microsoft's issues.