I know that file system access in WinRT is different (read: isolated), but I'm curious if we still have to worry about MAX_PATH, or has that restriction been avoided?
No, the MAX_PATH restrictions haven't been lifted - if you pass a path longer than MAX_PATH to a windows runtime API which accepts a path, it is still possible it will fail. But MAX_PATH is far less likely to be relevant, since windows runtime APIs typically operate on strings, and not on buffers of characters.
In addition, since metro style apps are typically restricted in the directories that they access, deep paths are less likely to be encountered.