I'd be interested in seeing how Flash/Silverlight performs on the ION. We have boxee running on an Atom box that we built for around $250, and video playback is generally pretty great, but Flash just flat out sucks. Hulu isn't watchable, using boxee, Firefox, Chrome -- whatever. It just sucks.
If ION can improve on that and boxee can support the GPU (for Apple TV it doesn’t, but that doesn't mean it couldn't for Windows), it might be a very viable little machine.
Having said all that, if the price is going to be closer to $400 (which I can believe), it might be worth considering a Mac mini instead. You can currently get the last generation (C2D 1.83, 1 GB of RAM (easy to upgrade), GMA 950) for $419 refurb from Apple.com or new for $499 on clearance. GMA 950 isn't going to win any awards, but that's what I have on my BlackBook and I play back .mkv and blu-ray content with ease.
The new mini has a faster C2D, and the 9400m. Yes, it's $600US (again, upgrade the RAM yourself), but it has DVI out, FireWire 800, built-in wireless-n, and a SuperDrive, which you aren't going to get on the Acer box or anything like it, without having to pony up extra for. Frankly, if I'm looking at spending $600 to get 4 GB of RAM and a large hard drive (which is what the upgrade will probably end up being), I'd kind of rather have the Mac mini and have a real processor and not an Atom.
We don't regret building out little mini-box (well, my fiance doesn't -- I kind of do), but in retrospect I wish we'd invested the $250 towards a mini instead of doing the Atom thing. If only because at this point, it absolutely can't be used as a real media box, only for music and AVI files stored on the NAS.
That isn't to say ION won't kick all kinds of ass, but I'd be sure to read up on the Flash and Silverlight benchmarks before buying, just because the Atom is SO puny (in all honesty, an Apple TV is far, far superior for playback and access).