The only way Apple would go back to Intel is if Larrabee kicked ass.
It's scheduled for a release in H1 2010, so inclusion in the Mac line is possible. However, nobody's sure what form factors will be available - it's possible (likely even) that there won't be a portable version at launch (probably coming in Q3 or Q4 2010).
EDIT: Actually, it's earlier than that. Intel now says "2009/2010" and gave details at a mobility conference, indicating that LB may launch with a portable GPU component. See:
http://arstechnica.com/hardware/new...oorestown-new-atoms-larrabee-in-late-2009.ars