Jesse has added support to Mesa and the X Server that will allow for the appropriate OpenGL/GLX extensions that expose syncing and frame counting capabilities (SGI_swap_control, SGI_video_sync, SGIX_swap_barrier, etc). This code though is currently housed in his personal Git repositories as the patches await review and feedback (mailing list message). This DRI2 synchronization and buffer swaps code is also dependent upon revisions to dri2proto and libdrm along with the latest DRM code that will eventually be pushed into the Linux 2.6.33 kernel due to its vblank event support.
Besides cutting down on tearing and other visual defects, proper synchronization of swaps can provide a smoother experience with a consistent frame-rate and can enhance the performance of some applications. More information on the technical end of this support can be found on the composite swaps Wiki page. This support isn't yet living in Git master, but it's surely on its way, which will benefit Linux desktop users.