I run into this as well. This is why I never let my HTPC go into sleep mode. It seems to be a hand-shake issue between the HDMI port and the HDTV. Doing a bit googling and I came across this thread in the XBMC Forum.Resolution/HDMI Sleep
Which lead me to Gefen HDMI Detective
Though expensive, it seems to cure what ails your setup. I might need to check this out myself!