People ARE donating with Bitcoins. This is their public donation address:
http://blockexplorer.com/address/1HB5XMLmzFVj8ALj6mfBsbifRoD4miY36vAs you can see, there have been a good amount of transactions after they announced that they're under a financial blockade. And that's only the public address, some people have most likely donated to a unique address.
from the wikileaks donation page bitcoin section:
For maximum security and anonymity, you can request a one-time Bitcoin address. To do so, please join our IRC and type /msg Bitcoin new. Our helpful bot will generate a new donation address for each transaction. Please make sure Bot is active on #bitcoin channel and has @ to the left of his nick.I assume most donations used that channel