This bug happens when you switch the ripple address to a bitcoin address after you already filled in the amount you're going to send.
As you can see, I switched to a bitcoin address but the client still treats the "20" as dollars instead of updating it to mean bitcoins. I must add that I have no trust lines with any gateways, so I shouldn't even have the option to send anything.
I can even press send, but of course when you click confirm it doesn't do anything.
All Bitcoin on the RIpple network are imaginary / fictional. A Bitcoin on Ripple is a "IOU" ticket. There are no real Bitcoins on Ripple.
You need to send the BTC to a Ripple address of your gateway, such as Bitstamp. Please, don't try to send to a Bitcoin address using Ripple. They are not real Bitcoins.
What's written above is wrong (likely motivated by http://www.reddit.com/r/Bitcoin/comments/1riyy3/hilarious_guy_tries_to_send_from_ripple_directly/) - by default Ripple-client will utilize the built in Bitcoin bridge provided by Bitstamp.
It will automatically sell XRP or other IOUs you own for BTC.Bitstamp that get redeemed and sent on the blockchain by them. The USD amout is an actual bug by the way, would be cool if this could be fixed. Expected behaviour would be calculation of paths and offering to use existing balances for the payment.