One of the big targets for Windows 7 is the 'netbook market. Netbooks are one area where Vista just hasn't taken off. Other than in a few places in Akihabara Japan, I've seen few places selling netbooks with Vista pre-loaded. In Australia, they almost always come with Windows XP or a Linux distro. Microsoft probably can't keep selling XP forever and would also be reluctant to abadon the 'netbook market to the Penguin, so getting the next version of Windows to work well with netbooks was probably a priority.
To get Windows 7 to install on my Eee 900HA, I plugged in my XBox 360's HD-DVD drive (not like I was doing anything else with it) and configured the Eee's BIOS to boot off it. Once I did that, the 32 bit version of Windows 7 installed with an absolute minimum of fuss on the second 60 gig partition that my Eee 900 HA is configured with. As others have mentioned, the Windows 7 installation routine is about as low impact as an installation routine can get. If things do get all borked up, you can roll back the installation at any time - though I'm pretty sure you won't find this necessary. All the Eee's hardware devices were recognized and Aero works with the built in Intel graphics card. Performance is great. I only have a gig of RAM on my Eee and with Office and IE open I don't have any memory issues. XP works great on the Eee and Windows 7 provides an almost identical experience performance wise. Booting is quick and I've noticed very few instances of the hard disk thrashing itself moving stuff in and out of the page file.
I'm never going to be able to play Crysis on it - but with the Eee's screen resolution I'm pretty much limited to games like Alpha Centauri and Galactic Civilizations II anyway (more fun than a DS!).
For those without a spare USB CD or DVD ROM lying around, it is possible to install the Windows 7 beta on the Eee using a specially configured USB device. The following blog post has one guy's approach: