I've seen this on the old and the new rom, but the common factor is that it will happen on my device and not my wife's.
Only real difference is that I have more mailboxes and get more emails.
Anyway, on the new rom, I noticed that when it happens, something messes up in power management and the backlight and display timeout are changed to their lowest setting of 10 seconds each. Not sure if having the display and backlight timeout the same is the cause or just a symptom, but I try to keep the backlight timeout at a lower number than the display timeout (eg: backlight at 30 seconds, display at 1 minute).
I had considered running a daily script to update the timeout settings but I got sidetracked recently by other issues that required me to get a warranty replacement.
Check if you have the same symptoms.