James has a good point, however, for those who do want/need to carry extra fuel another alternative might be the rectangular, plastic tanks used in small boats that have an outboad engine. Not only are they made for fuel and are very tough, but are in a great shape for mounting on a rear carrier. They are generally fairly low profile. You can get 10 or 20 litres capacity. If you wanted to get really keen, you could plumb it in to the bike's fuel lines quite easily. Also, Acerbis do plastic jerry cans with 2 mounting holes moulded through them which would ease mounting. Personally, I wouldn't carry extra fuel containers but would prefer to have a larger fuel tank. It's easier.
Tim ('91 R100GSPD)
The only baggage you carry should be in the panniers