The gains for Apple in doing this are obvious. Potentially faster processors would become available, and Macs could become cheaper as a result. But even if this does come to pass, don't expect Windows XP to run on a Mac and Mac OS X to run on a PC - Apple will no doubt implement technology that renders this impossible.
Any decision to go with Intel will require some rewriting of the OS X code. However it is highly unlikely that any changes will enable it to run on any Intel-based PC; Apple will ensure that it is hardware specific though hackers would doubtless be keen to get their hands on it.
Rumours of secret versions of Mac operating systems that run on Intel hardware have been circulating for years. They have been given some substance by former Apple employees who have suggested that it remains a fall-back option should the PowerPC platform fail.