The Russian mathematics portal Math-Net.Ru started using MathJax yesterday. Their database hosts 64 Russian mathematical journals and contains more than 80,000 mathematical abstracts with formulas in LaTeX. An interesting feature of the Math-Net.Ru implementation is the ability to toggle between MathJax typesetting and the original TeX markup by clicking on the MathJax button at the right of the navigation bar at the top of each of their pages.
To see an example MathJax in action on Math-Net.Ru, take a look at the entry for Checking the Numbers of the form [even product of primes minus 1] for Primality by E.V. Sadovnik.