OK. Given may (predictably) rapid dissolusion with XP on my EeePC - I love the way the OS is just so easy, hate the way that it's fat and uncontrollable (in my experience), I thought I'd just try Vista and see if it was actually possible and try and establish what does or doesn't work ... on my way to switching back to Linux.
I installed Windows Vista Business from a retail DVD ROM, via a USB DVD RAM Drive that has proven itself to be very useful.
The install itself takes about 2.5 hours and seems to go dead at a couple of points for period of up to about 20 minutes while the disk spins, but neither drive light comes on or the progress percentage increments. Stick with it. It will get there eventually.
Install to the 16GB partitioin and you'll end up with about 7 GB of OS installation on there.
Native screen rez is supported as are the hot keys etc. I needed to use the XP drivers for the LAN and ACPI hardware - go to device manager, choose the undignosed hardware and install the drivers by hand. This was sucessful and I was able to get the LAN and power sorted out quite quickly. I'll come to the WIFI in a moment.
A quick note about speed - not really any slower than XP. Overall the boot speed is about the same which given the fact that you end up installing onto a theoretically slower partition is quite impressive, and time to login-prompt is pretty well equal. After that, vista logs in faster but goes through starting all the services so you are looking at more-or-less similar time before you can actually get the start menu to respond or start an application.
OK. Wifi. Doesn't work. I've tried any native drivers (there don't seem to be any and the hardware isn't identified) an the XP drivers will install - if you diable the Wifi with the function key combination - but don't actually work or find any networks. I did some superficial research into this and there are other reports of people with similar experiences, and the only posted solutions are along the lines of 'use the XP drivers'.
Since the EeePC is a netbook, in my opinion it's mobility and therefore things like wireless are key to its usability. Without it, not so useful.
So all in all, impressed with the fact that Vista installs at all, pity that the lack of one driver is going to make it unusable and therefore rapidly replaced and quite suprised that speed wasn't a reason for uninstalling - this is an EeePC 900 don't forget ... thats a 900MHz celeron so processor power was always in short supply.