Despite my reading of HN, I've never really paid much attention to YC, but this data makes me ask:
Are all YC companies web apps? Are there no infrastructure companies, or other types of services?
There are many non-web app YC companies, including systems management products, mobile apps, installable apps, etc.
That said, you've specifically mentioned infrastructure, and I don't think an infrastructure company would be possible in the YC model. It would take a vast amount of money to start a new hosting provider or cloud provider, if you aren't building on existing infrastructure (which a few YC companies have done, building on EC2 and other cloud provider products, including recently acquired Heroku and Cloudkick). YC invests a few thousand dollars. Building infrastructure costs millions of dollars. Building software is what YC companies do, and the best place to build most software today is on the web.
WakeMate (http://wakemate.com/) developed a hardware device to track sleeping patterns. Although they end up hooking into a webapp for further sleep analysis, prototyping and production of a hardware device is sufficiently different from coding a website.
This should be a solid example that not everyone in YC is creating just a webapp that you can thrown onto a few EC2 instances in under 10 minutes.
