Anybody using foundation for apps already? Do WordPress themes for blogging count as "web apps", and should I use foundation to build WordPress themes?
WordPress out of the box isn't really built for single-page apps, or perhaps even apps in general. (To be sure though, I don't follow that community closely so I don't know what's up with it.) The distinction between sites and apps is often a fuzzy one, and it's arguably meaningless to users, but one good distinction is that a web app is closer to software. A portfolio, blog, or news outlet would be a website, while tools for chat, image editing, etc. are apps, and if they're on the web they're web apps. But there's a lot of overlap.
I helped build Foundation for Apps and I wouldn't recommend using it for a traditional WordPress site, although I expect the community will build themes for it eventually. However, FoundationPress is a great WordPress theme build on Foundation 5, which will continue to exist, and continue to be great for more traditional websites.