Tim Cartmell and his students are surely keeping it alive. I wrote an article on it last year for the Journal of Asian Martial Arts. Check it out on my articles page at www.threeharmonies.com

It is not as popular here in the west as in China, but it is far from "lost." Unfortunately many whom purport they do "traditional" Sun Taiji, are only doing an empty shell of the "traditional" form. Most are modern wushu derivatives, which I do not get since the Sun form is not that long to begin with. But to each their own.

I am happy to answer any questions you have if you wish to email me.
Cheers
Jake
three_harmonies@hotmail.com
www.threeharmonies.blogspot.com
www.threeharmonies.com