I ran accross this in the web site put out by the school where I took my motorcycle course. I have always gassed up while sitting on the bike, and never gave it a second thought. I think I'll get off from now on!
I put a link at the bottom that goes to the rest of the article.
"If you are on your bike while fuelling and spill fuel your clothes will become soaked. The tiniest spark from static electricity can and will ignite the cloud of fumes that you are the center of. This is not an uncommon source of ignition on boats with a fume filled bilge. At this point you will attempt to get away from the bike dropping it with the gas cap open, dumping large quantities of gasoline onto the fire at which point you are now in the center of a most spectacular fireball. Need I go further??"
Even if you don't go up in flames, gasoline spilled on the crotchatal areas will BURN your skin after 15-20 minutes..... remember what skin we're talking about here..... then a few days later the real fun begins when the skin dries and cracks and peels. Try explaining that one to your doctor.....
You don't want this experiance, get off the bike to fuel up.
I can't say that i've ever filled up while sitting on the bike. I'm usually going to grap one of those towel things to wipe down the tank, check chain tightness, plus go in to pay for the gas some i'm already off the bike. The last thing I do though is sit on it and get it in the up right position to check the oil.
Eh, I'm still gonna sit on it. I need to be able to tilt it off the sidestand to get maximum fillingness of the tank. If I go up in flames it's only a matter of minutes before I won't be around to care what happens.
Down in the South Sea's here it's against the law to sit on a bike and fill it... On the learner course's they teach you to discharge any static electricity that might have built up before you open the cap.
Saftey first, as my one legged, one armed Dad used to say, just before falling off a roof..
With only a limited number of days to live, work is a cruel joke..
I think I'll get off from now on! 
Try explaining that one to your doctor.....
