Ive pushed this film a full 2 stops (yes, mixing base, 1 push and 2 pushes on the same roll!) and then processed normally with very little image degradation.
Now, Jonathan Canlas has shown he is getting tremendous results all the way up to 3200 ISO.
See the fantastic images, and read all about it on his BLOG.
This film rocks. It's a little warmer than Fuji Pro 400H, expecially when overexposing. While the grain is finer, at least at 100% on screen, I have found it doesn't really matter in print. Once all my 400H is gone (got about 300 rolls in the freezer for weddings) Portra 400 will become my mainstay.