Not an acquisition deal as in "signing the contract this week" like with Skype but rather a deal for the opportunity to match an offer in the event that another company wants to acquire Nvidia.

From the article:

The deal gives Microsoft the exclusive right to match any offer for 30% or more of Nvidia's outstanding shares by a third-party." The agreement appears to be over 10 years old, dating back to the time of the contract for the GPU of the original X-Box.