It's actually possible to do this without even requiring a 3rd party for escrow. Two parties can perform a "cross-chain atomic swap", where special transactions are created that move both of the funds on both chains at once, requiring zero trust. This of course requires a custom client to create these special transactions.

I've been working on this exact thing for a while: https://github.com/mappum/mercury However, I'm still waiting for the altcoins to adopt BIP65, a Bitcoin patch that adds a new script opcode, which is necessary for cross-chain atomic swaps. Bitcoin recently adopted this change, so the altcoins should follow in the near future.