The folks at my local quilt shop routinely use 90-100 weight silk threads for quilting. They generally piece with a 40 or 50 weight, but if you're doing very close quilting, the very fine silk threads are incredible! I personally don't do that close of quilting, but I've seen their work and it is incredible. I think Dianne Gaudynski uses very fine silk threads too, and her work is incredible. The quilting shows up instead of the quilting thread.
My personal preference (since I am not a brilliant quilter!) is 50 weight So Fine or 40 weight Aurifil on top and SuperBob bobbin threads.
Pam