I visited RDU for a week to figure out if it was a place that I would want to settle down in. I was told the American Tobacco Campus was a hotbed for new startups, and of course SAS in Cary has a big presence. There's definitely talent in the area (State, UNC and Duke) and there's definitely potential.
One thing I noticed though was the relative lack of cultural diversity and city feel (the entire RDU area feels suburban and comfortable, and lacks "struggle" as it were), but I don't know if this adversely affects startup viability, so it probably doesn't matter. I decided it wasn't a place where I would feel comfortable settling in, being a city-person, but for many people who are turned off big cities, I could see RDU being a good middle ground.
Raleigh has plenty enough talent, from far more diverse backgrounds than Seattle, and just keeps growing. I don't need 10,000 engineers for a startup - I just need 10 really good ones and a pipeline for more. I've already got that network and I know the culture is better for me to find more, so I'm ready to go for it. It's a risk in some ways, but I gave Seattle a real try - it just isn't as uniquely conducive to building new things as people would like to believe.