I think maybe 2gb PC2-5300 667 sticks weren't out when the laptop was released? Either that or it's a typo. Lenovo now shows a p/n for such a stick along with the slower one. Either way, I looked through the thinkpad forum and people seem to be using the faster 2gb without any issues.

Thanks for your help! I ended up getting the Corsair version of this. It was marginally cheaper and a ton of positive reviews on newegg.