What is a "DVI" port? Which PowerBook G4 models have DVI ports? Which do not?

From Apple's own documentation, the DVI (Digital Visual Interface) port "for analog and digital video output enables a pure digital connection" to DVI-equipped displays and projectors. All systems released after April 29, 2002 have a DVI, mini-DVI, or dual-link DVI port with the exception of the PowerBook G4/867 12" (Al).

Details are provided on the appropriate specs page for each model, but also are provided below in this handy chart -- along with the maximum resolution supported on an external display -- for your convenience:

PowerBook G4	Ext. Display Port	Resolution Support
400 (Original - Ti)	1 (VGA)	1280x960
500 (Original - Ti)	1 (VGA)	1280x960
550 (Gigabit - Ti)	1 (VGA)	1280x960
667 (Gigabit - Ti)	1 (VGA)	1280x960
667 (DVI - Ti)	1 (DVI)	2048x1536
800 (DVI - Ti)	1 (DVI)	2048x1536
867 (Ti)	1 (DVI)	2048x1536
1.0 (Ti)	1 (DVI)	2048x1536
867 12" (Al)	1 (Mini-VGA)	1600x1200
1.0 17" (Al)	1 (DVI)	2048x1536
1.0 12" (DVI - Al)	1 (Mini-DVI)	1600x1200
1.0 15" (FW800 - Al)	1 (DVI)	2048x1536
1.25 15" (FW800 - Al)	1 (DVI)	2048x1536
1.33 17" (Al)	1 (DVI)	2048x1536
1.33 12" (Al)	1 (Mini-DVI)	1600x1200
1.33 15" (Al)	1 (DVI)	2048x1536
1.5 15" (Al)	1 (DVI)	2048x1536
1.5 17" (Al)	1 (DVI)	2048x1536
1.5 12" (Al)	1 (Mini-DVI)	1600x1200
1.5 15" (SMS/BT2 - Al)	1 (DVI)	2048x1536*
1.67 15" (Al)	1 (DVI)	2048x1536*
1.67 17" (Al)	1 (Dual-link DVI)	2560x1600
1.67 15" (DLSD/HR - Al)	1 (Dual-link DVI)	2560x1600
1.67 17" (DLSD/HR - Al)	1 (Dual-link DVI)	2560x1600

Can the PowerBook G4 operate properly with the lid closed?

Yes. According to Apple, the PowerBook G4 series can operate properly with the lid closed, which is useful if you wish to setup a "workstation" for your PowerBook G4 with an external monitor, keyboard, and mouse.

Apple also mentions that with the lid closed, "you cannot utilize video mirroring. . . [with] both a VGA and S-video source connected to the PowerBook, it shows an image only on the VGA device." So if you want the display "mirrored" on another display, such as a projector screen for a presentation, you will need to keep the PowerBook open. However, you can use an external monitor with the PowerBook G4 closed, provided that it is powered by the included AC adapter.