actually they need to convert them to bitcoin like yesterday, because what's going to happen is some of these banks are going to get payback in the form of freezing that 500,000 USD.
Just like they did to wikileaks
I agree that USD seem like a bad idea and recent history shows us as much. I would council Occupy-X to use PM's rather than BTC to store value though, and divided between a number of trustworthy participants to avoid a single point of failure. BTC is quite volatile (in terms of both value and legal standing). PM's can be converted to cash or BTC as needed.