Yep, its a ton of fun. Makes me smile when I see someone riding a regular bike.
I went ahead and put the battery in a backpack over the weekend. In some ways its better, like the rear of the bike seems happier going over bumps. However, the extra weight on me makes
my rear unhappy going over bumps (maybe I should get a squishy seat). The bike does seem a little less stable with the battery in the backpack, but maybe I'm just mistaking responsive for unstable. I'll leave it in the bag until I can figure out something better (or give up and just put it back on the rack). It'll probably help if I can find a place to mount my U-lock on my bike instead of adding its 4lbs to my backpack. Unfortunately, it seems perfectly sized not to fit anywhere on my bike.
Are you sure it uses cylindrical cells? All the pictures I've seen of Ping batteries show pouch cells. If they are cylindrical, I might be able to rearrange them and make a custom battery box in the triangle like guys do for Li-ion. I've also seen some setups where the battery is split between two bags that hang over the top tube. It wouldn't get the weight lower, but it would at least get more of it over the front tire.
I've been considering getting a smaller battery made out of higher C-rated cells like A123. Then I could have a 5-7Ah battery that would fit in the triangle and I could charge or swap out between trips. Li-ion would solve my problems, but there's the whole problem of me not wanting to burn my house down. :P