Tim00,
Thanks: the VM useage is 1.4gb for both Firefox and iPhoto, but my problem is actually the amount of free space showing on the hard drive, in Finder. Normally around 9GB, it steadily decreases as the Air is left on for any length of time, and had got to ~4GB a couple of days ago. This was coupled with slow perforformance, all of which went with a reboot (after a massive Force Quit session!).
All of which leads me to think that some sort of cach/VM is eating into the free disc space, and it seems to be worst when iPhoto is syncronising with MobileMe. This didn't happen with iPhoto 07, so maybe the 09 upgrade is the culprit?