I also have an x61t, and I, too, happily run Linux (albeit Ubuntu), but I would actually recommend sticking with XP considering your needs. (My tablet came with Vista, and while I wouldn't recommend that to anybody, I think it has the same handwriting recognition software as XP)
Previously-mentioned CellWriter is a grid-entry recognition system, meaning you have to write each letter in a box. Windows has great handwriting recognition that works on whole words. Using the Journal program, I could write notes down, then open up a search box and type in a query and it would find and highlight the handwritten text matching that query. Xournal, while great software, has nothing of the sort (granted, this is not Xournal's fault, but because of the lack of real handwriting rec.)
Also, even with Powertop I never found the battery life to be very good in Ubuntu (this may just be compatibility problems with Thinkpad models). Rather, do as someone else suggested and try a SSD drive. Also, turn off your wireless card when not using it (and while you're at it, other unnecessary services).
In the end though, a tablet is indeed very usable with GNU/Linux, but Windows will give you better handwriting recognition and perhaps more power savings.