Since this question seems to arise more and more often, here is a copy of Cimarron's post from the FAQs section.

http://www.taurusarmed.net/forums/index.php?topic=830.0

First Generation


The guns were marked only as Millennium
All were DAO

Second Generation


The guns were marked as Millennium Pro

Magazines were modified and will not interchange with First Generation models

Third Generation


The word "Pro" was moved from the left side where it read "Millennium Pro" to the right side where it reads "PT145 Pro"
The sights are dovetailed into the slide rather than screw mounted
The guns are now SA/DA rather than the traditional DAO

This FAQ was the product of the group effort of all the moderators of Taurus Armed.