I may have just raised a bug very much like this one; I missed it when I was filing the report with reportbug. My new bug is 625813<http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=625813>. I only experience a problem like the one described here when I wake up my screen after it as entered the screen saver and powered down the screen overnight but the symptoms sound exactly the same.
