My rules are still there, so perhaps Apple support speak with resource forked tongue.
Comparing an old (pre ML) rules file from 20th June with the ML one, the differences are:
The file name was: MessageRules.plist, the ML one is SynchedRules.plist. Both are in my V2 folder, but the old Rules have been copied (by Mail, not me) into the new SynchedRules file.
In the old file, for each item (Rule) the first line was Active (Key) String (Type) 1 (Value). In the ML file, this line is no longer there, so the first line is AllCriteriaMustBeSatisfied.
You can see these differnces in the "preview" part of Finder Show Info, if you have the two files to compare.
Thus, just copying and renaming the old Rules file probably won't work, because of this difference. Mail probably just makes a new file because the copied file isn't as expected. It should be possible to edit the old plist file to remove the "Active" line, and rename it to SynchedRules.plist (but see below). The top is slightly different, just below <dict>, as this relates to the Active line. There are a couple of lines at the bottom that aren't in the new file.
The "Active" line that has gone is now in a file called RulesActiveState.plist. This file is just a list of RuleIds from the SynchedRules.plist (like this:
AC6F6611-FF61-4AA6-9682-BA100EFAC233)
Doing this does sound rather tedious, but might be quicker than manually re-entering blocked email addresses etc. to replicate old rules. As plist files are in xml format, it should be possible to use a text editor to delete the unwanted "Active" line from the message rules list by Find & Replace All (with nothing). All this could then be copied to the "SynchedRules.plist. You would need to copy the RuleIds into the RulesActiveState.plist as well.
Make a copy of the files first just in case...