If you send it directly from MtGox or Coinbase or some other exchange that maintains the private key to the wallet holding your coins, you won't be able to sign it.
Damn good point!
For future incantations of group-buys and escrow stuff of this nature, that should be noted. I have sent BTC straight from my Mt. Gox account when I wanted to balance my funds differently. I didn't happen to do that here thankfully, but it was dumb luck because I didn't understand fully the signing process when I made my payment.
Had Instawallet not simply stolen/locked/lost all the funds that I had entrusted them with, I probably would have sent the BE payment from there. And I'm pretty sure they were not sophisticated enough to have signing capabilities. (BTW, Paymium aka paytuina aka bitcoin-central: Rot in Hell.)