Yep, I'm 100% serious. Right now, on my phone, is an alpha, proof-of-concept build of Portal. No, it's not official (it's definitely not supported by VALVe Software in any way), but it is tantalizingly awesome. Now, before you ask, we aren't going to link to the apk - it's from a pretty sketchy source. But if you look hard enough, you'll probably be able to find it out there in some of the darker corners of the web.
The thing is, you probably wouldn't want to play it, because there's actually not much to play. It's more proof of concept than anything, but it's a concept we very much like, and that basically seems to work - you listening, VALVe?