The current Mac OS cannot be used on a normal PC. The hardware architectures are completely different. However, when Apple switches to the Intel chip, it might be possible to get it on a PC. Apple will integrate a lot of security measures to try to keep this from happening, but I'm willing to bet that it will be hacked within a week of release.
The current OS X will not be able to run on standard PC hardware however the new Intel-OSX will be able to. Apple will release a restriction but some people have cracked it already.
I think this switch to intel chips is not going to be very good for apple. Think about it, why buy a $2000 mac machine when you can get a comparable pc for 1200-1500 and be able to install mac os on it. Macs are wayy more expensive than pc's, and I think that making mac os compatable with pc architecture will be a crushing blow for apple.