There will be no serious incentives on the 2015's F150's until the dealer lots are fully stocked with them. As far as I know, the Kansas City truck plant isn't even up and running with the new platform, so it'll be at least a month or two after that. What few 2015's are arriving at dealers are being snapped up by folks who have to have the latest and greatest at whatever price, so they don't need to put much of a discount on them.
From my informal look at Universal and Marlbrough dealerships, I'd say they are currently sitting at a mix of 80% 2014's to 20% 2015's.
Could probably swing a screamin deal on the 2014's, although that's not terribly appealing to me.
If I was buying a new truck this year, I'd probably be planning on picking it up in the fall.