When I installed F15 back in April, I noticed this error in /var/log/messages:
[ 25.005674] mcelog.setup: read: No such device
hasn't seen much activity, but with the newest kernel in koji, it appears to be fixed.
$ uname -sr
$ grep mcelog.setup /var/log/messages* | wc -l
$ grep mcelog.setup /var/log/messages | wc -l
Also, mce logging is finally working in F15. If you've been getting that error message, you might want to try the 18.104.22.168 kernel.