I also noticed that recently when placing an order from Mpix. I emailed them, and they informed me that they stopped using the Kokak Metallic paper and are using the Fuji Pearl paper.
I loved the Kodak Metallic paper, and I am now trying the Fuji Pearl paper. I haven't noticed a big difference in the few Pearl prints I have had made so far, but since I have only made a few prints with the Pearl paper it's hard to really judge it vs the Metallic paper. It does seem that the Kodak paper has more of a "metallic type of shine" to it, and that is why they named it Metallic. The Pearl does have the super gloss that the Metallic has, but it seems to be less in that 'shine' that the Metallic paper has.
One thing Mpix mentioned in the reply to me regarding the Pearl paper is that it is better at bringing out shadow areas. That can be good if you are looking to achieve more detail in those areas in your prints.