I'm not sure you need the visual component for developers - the hard part is getting a sense for what things cost, how long they take, and how people work.
Turns out those things are really hard to get a feeling for in isolation, but in comparisons, people are really good at it. I know I want an iPhone app; I come across http://www.flingmedia.com/, and the contact us form gives me his rates. Then you try to see if Brian's rates are valid; the pricing itself gives a marker of credibility. If Brian's rates are in line with what I see elsewhere, and his portfolio seems directionally suggestive enough that he can build what he says he can build, at the very least, I'm going to contact him. The conversation with Brian will tell me, as a client, if he'd be good to work with. Sure, it takes more work - you can't make judgments based so heavily on portfolios - but I really think Haystack would be really helpful to both clients and developers. (So, please, add developers!)
Semi-related, I'll bet you a dollar that people will be looking for local designers before they look at portfolios (rather than starting with portfolios) because you give them that option. For developers, I bet they'd do the same. You'll be able to track that relatively easily.