After a fairly decent Google search and spotting one or two of these questions on StackOverflow recently, I don't believe there is a C# library available.
However, OptiPNG is open source C code, so porting to C# is a possibility. Alternatively, pull the C code out into a separate library outside of an exe and P/Invoke into it.
There appear to be other open source libraries around, not necessarily specifically for optimization:
http://www.libpng.org/pub/png/pngcode.html
Apologies for the anti-climax, I've been after one of these too and I've come to the conclusion the current answer is one doesn't exist. I eagerly wait to be corrected.
