Wait, before you tell me I'm an idiot, here's why I think it's a good idea:
You can't get Autorun to autostart a program on a portable USB stick. But, because the U3 environment sits on that little read-only partition it autostarts the U3 Launchpad when you insert the stick.
... So what? Well - You can configure the U3 Launchpad to autostart any U3 application on startup.
Therefore, you take advantage of U3 (which kind of sucks in my opinion) and use it for something useful - Remove all the existing U3 apps from your stick and just add the U3-enabled PStart, and set it to autorun. Then - Bam, the flexibility of using any portable app you want, with the convenience of an automatically starting launcher.
Unfortunately, PStart is closed source... I sent an email to the author suggesting the idea, but haven't heard back yet. If anybody here has any pull with the author, why don't you drop him a line, too?
Assuming you don't think my idea is idiotic. Heh.
I went and downloaded the U3 development kit and figured out how to do it. It works great! I read over the instructions and figured out how it works - It's really not that complicated.
Unfortunately, http://www.pegtop.de/start/ says, in part, "Of course you are allowed to link to this website, but you must neither publish this product elsewhere nor provide a direct download link without permission."
So, I can't link the PStart.u3p file here, but... Still, it's really not that complicated.