Users on the Apple Support Discussion forums are reporting problems with third-party RAM upgrades, noting system instability and lock-ups. Even users who have purchased RAM from providers that pride themselves on Mac compatibility such as Crucial and OWC are seeing the same issues. It seems that the only solution is to take out the third-party RAM and use Apple-branded RAM. International users with no access to an Apple store are currently out of luck.
Of course, Apple will only officially support their own branded RAM from Samsung; upgrade kits of this sort are apparently working without issue. Unfortunately for many, however, acquiring these modules has become somewhat of an act of futility as Apple's online store says the upgrade kits are unavailable for owners of the 2.0 and 2.4 GHz MacBook.