That's an interesting question, though. Mac O.S. is basically its own flavour of Unix. Because of this, it makes sense that Iphone O.S. is some derivative of Unix. Since Android is based on the Linux kernel and uses a sort of Linuxy userspace, it might be possible to reverse engineer Iphone O.S., or somehow get the source code (good luck doing this!), patch it and compile it for an A.R.M. prcessor and make a Nexus One boot into it.
I doubt that anyone will really bother, though; there is so much work involved, and you can already buy a Iphone.