I don't know if my problem is related to this, but I'm asking for advice just in case. After resuming mediaportal from standby, my desktop is 1920x1080, but mediaportal is 1600x1080 (or something like that, I have a fullsize desktop, but mediaportal only fills 4/5 of it from the left side out). The registry hack helped on my previous htpc, but that started as 720x576. x & y values in the registry are set to 1920x1080, but mediaportal always has a screwed up aspect ratio. I'm running 1.2 beta by the way.
If you are on HDMI <--> HDMI connection, yes it looks like it is the problem we are talking about here.