Finally decided to take the plunge and build an Atom 330 / Nvidia ION based Home Theatre PC. Had been thinking of the Revo 3601, but not sure it's out yet, and also will feel a bit happier having chosen the components and built it from scratch. Total cost would have been about £250, but with the addition of the Di Novo Edge, it's closer to £350. (But the Edge is just too cool an option for a living-room based media computer!) On the downside, it's going to be slightly bigger than a Revo, and probably not totally silent, but it should be a bit better spec'd.

Asus motherboard: http://www.asus.com/product.aspx?P_ID=xrR7wto9Z5BL42aU&templete=2
Compucase Mini ITX Case: http://www.compucase-hec.co.uk/pccase_mini-itx_8k.htm
Logitech diNovo Edge: http://www.logitech.com/index.cfm/keyboards/keyboard/devices/192&cl=US,EN

Hitachi 500GB Hard Drive SATAII 7200rpm 16MB Cache - OEM 139257
Compucase Mini ITX Case - With 120W PSU and Front panel USB2.0 HD Audio 156186
ASUS AT3N7A-I NVIDIA ION with Intel Atom N330 Dual Core Processor HDMI VGA Out 8 Channel Audio Mini ITX Motherboard 173797
Crucial 4GB Kit (2x2GB) DDR2 800MHz/PC2-6400 Memory Non-ECC Unbuffered CL6 Lifetime Warranty 142403
Logitech diNovo Edge Bluetooth Rechargable Wireless Keyboard - USB 119977
Belkin HDMI To HDMI Cable 5m 79224

All the hardware is from eBuyer. Had been looking at a mix of Dabs and Amazon, but eBuyer ended up cheaper on just about everything. Also sent an email "query" to Dabs to see if their Value ITX Case takes a 2.5 or a 3.5 internal HDD, but just got some generic response explaining why they think it's a good idea to have an unbranded Value range. As it turned out, found a similar case for £20 less.