At least, to my knowledge, this is undocumented. Look at this:
In the same way that you can delete certain items on the iPhone, you can delete individual videos. You swipe your finger from left to right over the list entry and you get this Delete button. If you click it, you are asked to confirm, then away it goes.
Strangely, you can’t delete songs. I wondered what could be the purpose of manually deleting content until I thought about the WiFi iTunes Store. You could easily run out of room for purchases, so you can throw out the big stuff to make way. Also useful if the contacts you enter balloon out of control. 😉
Speaking of contacts, you can’t delete them in this manner, although you can delete them if you edit them first. You can’t edit nor delete calendar events.
I wonder what else I’ll find.