In the last big update few days ago, there was a bug slipped the team in which Intel GPU users were unable to start their desktop after upgrading to the latest XOrg and Mesa packages. Many people reported this on LQ, G+, and also on AlienBOB's blog. Several people came up to help and Ponce finally gathered all the information and push a new build on his repository.
The packages is good and i have tested it on my workstation which also uses Intel GPU and it does work. Few hours later, Pat pushed the new update for xf86-video-intel and some other related packages in -current tree. This issue should be solved by now and you can safely upgrade your packages using slackpkg as usual.
Happy Slacking and Testing :)