CritterNYC - 2:00 pm on Jul 21, 2010 (gmt 0)
I am not a lawyer, but have done a lot of open source work. The parts of the theme that would be a plugin and directly interface with WordPress (ie, API calls back and forth, data sharing back and forth) would appear to be covered by the GPL and should be GPLed.
However, any images and CSS files do *NOT* need to be GPLed. These do not interact directly with WordPress other than being referred to in HTML, so they're basically combined in the browser.
You can have open source code that opens up and displays closed source / copyright-protected images without any issue of GPL virality.