Is WPF on Linux (already) possible?

You'll have better luck working with Moonlight, which targets the Silverlight API, which is a subset of full WPF.

edit: Sure, Silverlight isn't "intended" for the desktop, but there's no reason why you can't embed a silverlight engine in your application. It's been done before, such as for the Mac NY Times Reader

more edit: see Miguel's post on Standalone Silverlight Applications

Update: Since people keep upvoting this, I want to point out it is long since out of date. Mono got acquired by MS years ago, and their posture regarding open-source has changed, so consider this post obsolete. (As obsolete as the WPF framework itself, heh).

Mono is in a bit of an uncomfortable position when it comes to Microsoft APIs such as Winforms and WPF. A subset of the .Net technology is an ECMA standard, but free implementations of these APIs are probably on shakier legal ground. I believe this was a large factor in the covenant between Novell and Microsoft, which is good for Novell customers. But people who use Mono that aren't customers of Novell aren't protected. For this reason a lot of people in the F/OSS community look askance at Mono despite its technical merits.

For this reason, Gtk# will always be preferred, since it is truly Free. Many people consider it to be superior to Winforms anyway. As far as WPF is concerned, it will almost certainly be a low priority for Novell. They may implement it eventually, but I would expect Moonlight to be the closest you could get for the forseeable future.

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Since posting this, Microsoft has extended their covenant to anybody who implements the ECMA 334 & 335 standards.