That screenshot is 640x480. The OMAP3530 is obviously capable of going higher than that, but after loading an MDF the display manager still wasn't letting me change mode so I guess there's another bug somewhere.
There isn't a UK distributor for the beagleboard that I'm aware of, but Digi-Key do provide good service (Express UPS delivery so the board arrives in a couple of days even if it's shipped from the US, and mine came with free shipping - although I can't find a reference for which orders qualify for free shipping). However they do have a downside in that the order process on their website isn't the most consumer friendly, and judging by newsgroup postings they occasionally have hiccups with their ordering software that flags the beagleboard as being export prohibited, leading to lots of cancelled orders and confused customers! Also you do have to be prepared to pay the delivery man VAT on delivery (about £20).
There's also special computing as you mention, which is the only place I know of that supplies a case. Just be aware that the beagleboard support page says that they only accept RMAs on boards purchased from Digi-Key, although I'm not really sure if that's true or not.
There isn't an idiot's guide to setting everything up at the moment. At the moment it's still very much developers-only, especially since there's currently no code to load/save the CMOS RAM state, and the beagleboard doesn't have a battery-backed clock so you'd have to reset the date and time each time you turn it on (Although there are rumours that the next revision of the board will have a battery for the RTC). I'm not really sure what ROOL's plans are for distributing the ROM image - they do have a download for an older version on the website, so maybe in a week or two once the code has settled down they'll put up a new image that's more suitable for the general public. Until then if you wanted to run it you'd have to have the C/C++ tools to be able to grab the source and make your own build.
At the moment I guess buying a beagleboard could be an atttractive option for people looking for a speed boost over a RiscPC, but there's not much point in Iyonix/A9home owners rushing out and buying one unless they plan on helping out with the port or if they have some particular piece of software that needs testing. It will still be a while until all the hardware features are available (e.g. the video overlay and hardware floating point), and a bit longer for software to start making use of the features, by which time there will hopefully be more consumer-friendly devices available like the Touch Book or Pandora.