I wrote about this a while back, but it seems that Digidesign has finally updated Pro Tools for Leopard:

Digidesign is pleased to announce the availability of Pro Tools® 7.4.2 software, which provides Mac OS X 10.5.3* (Leopard) compatibility to Pro Tools HD®, Pro Tools LE®, and Pro Tools M-Powered™ users. We’d like to thank our entire community of Mac users for their patience as we worked to bring a fully tested and reliable version of Pro Tools to Apple’s latest platform.