I don't know if it's more frequent in 2.6.6, but I've noticed it on occasion in just about every Linux version I've ever used. The problem is that /var/run/utmp gets out of sync with reality. I would check the permissions and make sure that they haven't changed. Slackware defaults the permissions to 664 with an owner of root and group of utmp, but other flavors of Linux may do it differently. If you're reasonably sure that nothing besides the kernel version has changed, it probably is some sort of bug with kernel 2.6.6. On the rare occasions that it has happened to me, either rebooting, or typing "cat /dev/null >/var/run/utmp" may help. The latter will make the system think nobody is logged in, so it's not perfect either.
