If you use Mail with any sort of plug-ins that utilise the Bundles interface, make sure you remove/disable them prior to installing Tiger. Plug-ins such as:
1. Growl Mail
2. HTTPMail plug-in (for webmail like Hotmail)
3. Mail appetizer
Unless you do remove them, Mail fails to import any of your old mail and this could resort in hair loss
Take this from someone who did it the hard way