I think that the cross-promotion is 80% of the value of C77 and IDSA working together. IDSA has tried to do somethings that C77 does much better: Coroflot job board and portfolios, C77 blog, C77 discussion forums. However, IDSA does somethings that C77 can’t do: organizing conferences, printing a great magazine, lobbying and publicizing design to a wider audience (I guess C77 does that last one, but in a much different way).
If IDSA improves those core competencies by directing people to C77 who need the other stuff, it’s a huge win-win. Also, long term, the rising tide will raise both boats.