I just got a message from a friend I hadn't heard from in a while. We were catching up when he mentioned a startup idea he had been thinking about. My first thought: "Oh no! Please don't pull the '... and I just need a coder to do it!' bit on me!".
Anyone who did an undergraduate degree in CS knows how it is to be bombarded by emails to the department listserv from business school students looking for just that - and you also know how dismal their success rate is in getting someone to help them.
But my friend, thankfully, didn't do this. Instead, he described his idea to me, talked about why he thought it was a good one, and then asked for my feedback. By doing this, he showed that he respected my opinion with regard to the business idea as a whole, had done some serious thinking about the problem space, and didn't have delusions of grandeur about the potential of his business.
At the end, he casually mentioned that he'd like my involvement in his project, and I didn't dismiss the idea out of hand like I usually would. On the contrary, if I didn't already have too much on my plate I probably would have offered to help him prototype the site. And while he didn't get someone to help him with his startup, he did (hopefully) get some helpful criticism that will improve his chances moving forward.
I think this is the right approach for trying to attract a technical co-founder. You're looking for a partner, not an employee. Treat them like one.