If I put my laptop (Thinkpad X61 tablet) to sleep and then resume it, the screen is always dimmed (to on-battery level or so) when it resumes, but if I hit Fn+(brightness up), the OSD claims it's at full brightness. But if I do Fn+(brightness down) once and then Fn+(brightness up), it gets back in sync and fixes things.
What's the output of "xbacklight" when you resume without pressing any buttons?
100.000000
This seems to have fixed itself. Given the xbacklight output, I'm going to assume it wasn't actually g-p-m's fault anyway, and was some driver bug that is now fixed.