When in DVD Player simply press menu and it brings up some on-screen options, 'Eject Disc' is about the third or fourth down, highlight and press play.
Well, when a DVD is playing, you hit 'menu' once to come to the DVD's menu. Hit 'menu' a second time to get to the Front Row 'DVD menu', which has an 'Eject Disc' option.
Or, if the disc is in, but not playing, just go to the main 'DVD' menu item, and the menu will have 'Eject Disc' as an option.