Reportedly last time it was because Nvidia supplied a lot of GPUs with bad bump material and/or crappy solder, causing frequent and early failures that led to Apple recalls. Apple removed Nvidia from its entire lineup over the next two years.
About a year later, AMD got on Apple's bad side when they revealed a partnership a day or two prior to an Apple event. Rumor has it that Jobs got pissed, removed all mention of AMD's partnership from from the event slides, and started to bring Nvidia chips back in.
And now Nvidia is back out. I don't know why they are doing it this time. If anything, I consider the current Nvidia lineup to be much more power efficient and run cooler than AMD, things that Apple would normally value quite highly. People have speculated that AMD is offering rock-bottom prices.
So anyway, it's a lot of back and forth. I don't read too much into it and I don't think it's forever.