The VGA & component cables of the current 360 can already do 1080p. Adding HDMI isnt much of an incentive to get another 360 but the smaller/cooler chip is a good incentive if noise, heat and power consumption are consideration. HDMI's useful if you want less cable clutter to a single wire, want a purely digital signal and if HDCP is already required.
Based on this
article the PS3 doesnt have a built-in hardware scaler making em suck on HDTVs without hardware scalers.