Update 6/1/09 - Piaggio has commenced a recall for this problem. If you have not performed this fix already, your dealer should do it at no charge. The factory instructions are to replace the hose and install hose clamps.
#2 Standard Blade Screw Driver
NAPA submersible 5/16" fuel line hose (part # H209)
NAPA high pressure hose clamps (part # 705-1226)
2 ea 10" tie wraps (optional)
The inspection and possible repair consists of pulling the fuel pump/fuel tank sender/fuel filter assembly out of the fuel tank and seeing if there are hose clamps and a black hose installed on the fuel pump. If not, it is a simple matter of replaceing with the proper hose and installing hose clamps.
There is no need to pull the entire assembly up and out, I would just hold it up with a string or weak bungee cord attached to the handlebars while working on it.
Look at the hose below the white lid. There should be two hose clamps on a BLACK hose. If your hose is clear, as in the picture below, it must be replaced. If the clamps are not there, or you have a clear hose, YOUR SCOOTER IS DEFECTIVE.....so install some! Buy metal hose clamps for a ½" (12mm) hose. Unscrew them completely and wrap them around the hoses. They should be quite secure, but be careful about over-tightening them. You are wrapping around plastic fittings. Your fuel tank sender assembly should look like Figure 4.