it could also be they have booked marked it was the www, and it should not be, or the other way around.
Hope that gets fixed in V8
Well that'd be a cross domain cookie issue; the only real way I see that we could "fix" this would be to force every connection to one URL; which a lot of communities use multiple domain names so that'd not really be a good "fix".
A real "fix" would be to redirect all users accessing the non-www version of your site to the www version (or vice versa; whatever floats your boat).