I have considered offering free hosting to YC companies. Most of them seem to be a bit, ah, brand conscious (thus going for slicehost over linode) so I haven't bothered to push the idea.
But yea, if any YC company needs free hosting (and really, hosting is so cheap these days that it's all pretty much free) I'd be happy to give it to them. I'm younger than linode, but I've been renting Xen VPSs since 2005, which makes me older than slicehost, I believe.
I'm not asking you to go exclusive or anything (in fact, I think that's a bad idea. don't host all your stuff with the same provider.)
That said, I'd be a little surprised if any of the YC companies have to pay for hosting. Many, many ISPs would be happy to give away a little bandwidth in exchange for some publicity.