The Avalon team has been working super hard at getting a CTP out the door. Well... it's here!!
What's amazing is that this drop aligns with the WinFX platform's long-term goal of shipping down level. In less PHB-ish terms: it runs on XP! Just a few months after the announcement that things would go down level. This means there's a very low barrier to entry for tinkering... i.e. no download and installation of the entire Longhorn OS, just a set of binaries with a cute little installer and the SDK. Unfortunately, you need to be an MSDN subscriber to get yer hands on it at this point.
The CTP runs on top of the 2.0.40607.51 stack, otherwise known as Whidbey Beta 1. And it's compatible with the Express SKUs as well. However, as JasonZ mentions, you're not going to get very far trying to get it to work with the recent Whidbey CTP. Our story here should get better over time.
There's already some buzz going around the MS blogosphere... ChrisAn has a pretty cool demo app which uses ClickOnce for deployment. I'm working on shoring up my recently promised application and will try to post something--now that a semi-comparative set of bits to what it was built on are available!
Update: Just noticed that RRelyea has a great post describing this drop.