With MySkit and the like you can create a skit (sound and/or animation) and connect it to one of a group of predefined events (back touched, startup and so on). For example, play a certain sound on startup. The same thing goes for shadowing, you can put sounds and motions on an SD card and your pleo will use those instead of the original ones. But that's all you can do. You can't, for example, tell your pleo that certain actions should make it happy, or to take a picture when it spots its leaf. But those types of things should be very doable with the PDK and a bit of time and programming knowledge. You should also be able to connect actions to whole new conditions, like a loud sound from the left.
It all depends on how much functionality Ugobe opens up to us outsiders of course. If they go for maximum open-ness, the PDK would allow us to do anything they themselves can do with a pleo and more. To go a little wild, I could imagine a pleo with a wireless antenna hooked into the USB port, controlled by a personality (an AI) running on your computer thus having much more power and flexibility. Or the football playing pleos we mentioned before. Or my "blogging" pleo.
Myself, I've only tried a slight bit of skit development. And of course, I've shadowed the startup sound so Gob makes a Macintosh "bing" sound when waking up.