Of course the downside is that I expect the monitor will do a crap job of displaying the signal from the PS3 :P
If it's a good monitor, you won't have a problem. I just got an HDTV myself, but before that I had my 360 hooked up to my 24" monitor via the VGA cable and it looked great. For the PS3, if you get an HDMI to DVI converter the signal will look just as good on the monitor as it does the TV... just make sure you get a monitor that supports HDCP over DVI.