Many Powerbook G3 and G4 owners have noticed that the LCD screen often shows evidence of keyboard key contacts on the screen. Usually this is due to oils from your fingers transferred from the keys onto the screen when the lid is closed and can be removed with screen cleaners. Some owners however have reported marks that cannot be easily removed, although I think these cases are rare overall. At first I thought my PB G4 didn't have this problem, but looking at the screen from an angle showed there were outlines of the keys present on the screen. The severity of the impressions will vary depending on individual PB manufacturing tolerances, pressure applied to the lid when closed, etc.
In the past I have used a cloth over the keyboard to prevent key contact with the screen, but recently received one of OWC's PowerBook Screen Protectors. The protector is a thin piece of soft leather that looks a lot nicer than my home-made solution. Another company recently announced a set of replacement key caps called KeyBorg that are lower profile to avoid contact with the screen, but they're more expensive ($79-$89) and not currently available for the PB G4, although they are taking pre-orders for the PB G3 series and original iBooks. Personally, I'd rather not remove the original key caps but some folks may feel differently.
The photos below show the OWC screen protector as shipped and installed on a PowerBook G4 and PowerBook G3.
OWC notes the protector fits any Apple PowerBook or iBook made from 1998-2001 (Wallstreet, Lombard, Pismo, Titanium, and all iBooks). The price as of Oct. 4th 2001 was $12.95 plus shipping and includes a packet with IKlear's screen polish/cleaning cloths (single use) but as of early 2003, the prices are a few dollars higher. They later added more models also - including one that covers the entire PB G4 15" top surface, not just the keyboard.) For more info see OWC's Notebook Accessories product page.
OWC is a site sponsor and their xlr8yourmac site specials page lists other PB products like the CoolPad, etc. as well as RAM, drives and other upgrades.