The basis for the idea is threefold:
1. There is a lot of goodwill in the startup world (recent Offer HN posts are evidence). Many startup people I know would go out of their way to help their peers as long as they are not direct competitors.
2. Startup founders have a lot of human capital: they know and on a daily basis talk to many people who are potential clients not just for their own companies, but for their peers as well.
3. The most complicated part of business for most startups is sales (customer acquisition).
The idea is to put up a website where one could register a startup, provide basic information: name, product description (15-second pitch), target group. Startups with matching target groups would be informed about each other (I am thinking regular emails), and with some luck and good will would refer potential customers to each other.
Yeah, the idea is hopelessly naive, but maybe it could just work out?