Several major airlines now use the EmPower system. You will need an adaptor like the one you mentioned to utilize. Some airlines only have the sockets in business and first class, but many are extending to coach. It pays to check with the airline for your specific flight ahead of time. I think VST also has a PDF you can download that lists the planes. A lot of international flights have the sockets.
The VST adaptor runs about $50 for the G4 specific cable. You might also want to consider the interchangeable systems like the one from Xtend Micro called the PowerXtend. For about $100 you get a universal system with interchangeable cables for various laptop models. In addition to the EmPower adaptor you also get a automobile cigarette lighter adaptor! E106x is the model number for the Titanium kit.
www.xtendmicro.com
[ 01-05-2002: Message edited by: jwtseng ]