the biggest problem with apple enter the market is the fact the hardcore Apple guys for the most part are not gamers at all. Microsoft always has had an in road with gamers, so it was not too much of a stretch. I think it would suit apple best to start as a 3rd party Dev, build some AAA IP's and then make a handheld or Console once they have a solid 1st Party following.
I honestly feel that Apple is a fad. Most of the people I know who want one want one because it is "cool" and sharp looking.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut