If the two companies cannot reach an agreement, NVIDIA would continue to provide chips for models that currently use NVIDIA, but Apple would be likely to drop NVIDIA chipsets in updates to their product line, particularly in iMacs and MacBooks currently based on Intel's Nehalem architecture.
A significant factor in the disagreement is the way NVIDIA handled the graphics failures of MacBook Pros carrying the GeForce 8600M video chipset, which had a tendency to overheat and eventually stop working. Apple had to extend the warranty on MBP
The relationship between Intel and NVIDIA hasn't exactly helped, either. Both businesses filed opposing lawsuits over NVIDIA's license to make mainboard chipsets with their own internal memory controllers. If Intel wins, NVIDIA could not make another chipset like its GeForce 9400M model that supports Core i7 processors, and would oust NVIDIA from Macs by exclusion.
Neither Apple nor NVIDIA have publicly spoken on the matter so far. Apple does have a history of severing relationships almost without warning, as they dropped ATI (now AMD) from Power Mac G4s after the company revealed Apple's plans ahead of a Macworld keynote address. However, if Apple does indeed drop NVIDIA, they may have to return to AMD in order to maintain their current graphics standard.