Tested Office Live on Windows 2000 with IE 6. Document collaboration features do not work. But I was happy to see I could build a web site in Small Business Live using my Windows 2000 machine.
But since document collaboration is the killer app of SharePoint I would imagine Office Live, which is SharePoint in the Cloud is still not going to be of much value on older Windows 2000 machines. I have found there are a disturbing large number of such machines still in use. I have tested SharePoint installations on this box and they have worked. So Office Live is blocking certain OS configurations like Windows 2000 or Linux.