The iMac treats it as a totally separate display so it will run at it's native resolution. It doesn't matter if it's widescreen or not, the internal display has no bearing on what the external display is doing.
You can plug a 23" ACD into the iMac (any current iMac) and it will run at 1920x1200 whilst the internal display will run at it's own native resolution.
You could plug a 20" 4:3 display into any current iMac and it will run at it's native resolution (most likely 1600x1200) and the internal display will run at it's own native resolution.
The ratio and resolution of the iMac's internal display has no bearing on what the external display will do - both of them are separate and will run at their native resolution.
Same deal for the MacBook, MacBook Pro and all iMacs.