Can't wake monitor up from suspend/hiberate

[freem@n]

The HDMI to DVI cable can't be the problem because I'm using one too to connect the Pi to my monitor and it's always possible to wake him up even after a day or so. In fact the monitor doesn't even turn itself off completely but goes in some kind of standby mode (so I turn it off manually to save energy).

So it seems that your monitor turns itself completely off, right? this brings up the question: Is your Raspberry Pi connected to the TV via USB as a power source or are you using a separate consistent USB power source (which?)
if the Pi is connected to the screen via USB it would be logical that the TV kills the power to the Pi immediately when it goes to standby.

Apart from that you can turn the screensaver off directly in the XBian-GUI (and even in normal XBMC). Just go into Settings -> Appearance -> Screensaver and choose "None" as Screensaver Mode.
also uncheck the option "Use dim if paused during video playback"