I'm not sure how long this deal will last, but Amazon.com has Crazy Eddie pricing on the Windows Home Server-based HP LX195 MediaSmart Home Server (codenamed "Boxster"). This isn't an ideal WHS machine for a number of reasons--you can only put a single drive inside it, so all expansion is external, for starters--but this is pretty much the cheapest way to get into a high-quality WHS machine I've ever seen.
Check out my overview of the HP MediaSmart Server LX195 for more info.
Thanks to Alex Bowman for the tip.
Update: This more expandable Acer unit ($300) might actually be a better deal, given the expandability. Thanks to Michael Creed.