[Update: According to links provided by commenters, it can. Seems early launch day users just didn't activate USB charging, which is an option on the Vita. So, we can scratch this negative off the list, which is nice.]
With the PS Vita out in Japan, all the details Sony kept from the public are spilling onto the Internet. One interesting bit of news is that the system will not be rechargeable via a USB port, requiring a wall outlet in order to get its battery life back.
The Vita-to-adapter cable is USB, but you won't be getting any juice to your system without a good old fashioned socket. Considering Sony's handheld is powered by Spider-Man's radioactive blood, it makes sense that a USB port might not be adequate enough to bring it life.
Hardly a major dealbreaker, but as someone who charges almost everything via USB if he can do so, I can't say I'm clapping my hands with gratitude. Ah well.