Pro Tools is the industry-standard audio creation and production software used in the music and entertainment industries. Traditionally, Pro Tools takes its sweet time to be updated to work on newer OSs, but today Avid has released a Pro Tools 9.0.5 beta, which adds support for OS X 10.7 Lion. Note, however, that even though Pro Tools 9.0.5 is officially qualified to work on Snow Leopard systems, that's not the case with Lion. Hence the "beta" status. If your livelihood depends on Pro Tools, you probably shouldn't upgrade your production workstations to Lion and version 9.0.5.
If you intend to upgrade, note this warning and advice from Avid. When Lion is installed on a system with Pro Tools, the OS will disable some Pro Tool plug-ins. Given that, Avid recommends against updating a Snow Leopard workstation that already has Pro Tools installed. Instead they suggest Pro Tools users create a new partition on their hard drives, then install Lion from scratch, and then install Pro Tools 9.0.5.