Assume that rules from Panther are to be imported. In order to import old rules, rename the /Users/.../Library/Mail/MessageSorting.plist file to MessageRules.plist. Then, open up the file in a text editor and put the outermost <array></array> portion inside of a <dict></dict> with the key "rules." Also, remove each of these:
<key>Version</key>
<string>2</string>
from within each of the old rules, and put just one entry outside of the outermost <array></array> portion of the file. Make sure to indent uniformly:
<dict>
<key>rules</key>
<array>
...
</array>
<key>Version</key>
<string>2</string>
<dict>
[robg adds: I haven't tested this one, and just chose to recreate my rules after migrating.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050511230631364