I just bought an Acer 1810TZ for £440 off Dixons (with about £15 cashback from quidco). It's for the ULV 1.3Ghz C2D, 4Gb of Ram, ~7 hours battery. The only downside is the Intel integrated graphics. This is in a 11.6 inch case, perfect for dumping in my bag for use on call or for long journeys.
I'm thinking this machine with an ION2 would be almost the perfect ultra portable. Maybe an i3/i5 would be nice too, but i'm just greedy. The C2D is easily good enough for my purposes.
Happiness is a positive cash flow.