I have been running XP on my P-6860FX for over a week now and I am very happy with it. There are three 'unknown' devices in device manager, but a little Google research indicates that these are ACPI extensions for Vista's 'direct application launch'; XP does not support this so I am ignoring them.
I have been testing the battery under XP and under normal use (browsing, downloading, blogging, email, etc.) I am getting a little over 2.5 hours per charge. Not the best battery life for a laptop (albeit it is pretty good for a 17" laptop) but it is much better than the 1 - 1.5 hours I was getting with Vista!
I also found that the Gateway Camera Assistant works in XP.