i think the vast majority of people who owns a gmail address or an address with any other provider won't change their email to a facebook address, why?
because it's a lot of hassle to notify all of your contacts (i'm talking about not just friends but anybody who have your email address, even on offline channels, lot of formal places), not talking about the lots of websites where you're already registered with your email, most likely you won't start logging into 100+ sites, forums just to change your email address
however it's also true that gmail can be forwarded (yahoo has a paid forwarding service), but a mortal internet user is not really aware of this
so in short: i just can't see this happening, gmail is way too big even for FB and what is most important, gmail was first
it's nothing like how FB beat Myspace, changing an established email address is a totally different kind of animal