Double-spending is not yet much of an issue because very few merchants are vulnerable to it. You have the BlockChain.info mixer, and SatoshiDice and its clones. The latter easily respond by spending double-spend attempts 100% to fees, and proving their honesty after the fact by keeping records of the double-spend.
From what I've heard a lot of merchants who have physical contact with customer (e.g. a restaurant) accept payments with zero confirmations. Each time this is mentioned on reddit, somebody says "don't worry, you just need to wait a couple of seconds". Yeah, right...
Is it still possible to make transaction which is unlikely to be included into a block? Perhaps just low-priority one (freshly sent coins) and no fee.
This will likely give you at least a few hours of opportunity to pull off a double-spend. So you can start once you're already far away from this merchant physically.
Also, I doubt that merchant will notice that money disappeared and associate it with you.
Once this patch is ready I'll try to help with the front end. I'm currently working on web wallet, so double-spending might become shockingly easy =)