Warning against preupgrade

[Kevin Bowling]

2 out of 5 failures.  F10->F11 is completely unusable for any kind of 
"uncommon" setup, i.e. LDAP login, Linux RAID, Xen DomU.  Anything other 
than one IDE hard disk with default layout really.

It worked well in the past, it doesn't now.  Just a heads up since 
nobody bothered mentioning this beforehand.  It should probably be added 
to "common bugs" and in a big banner on the release notes.