I would love it if this widget eager-loaded the directory structure for a couple levels (doesn't even need to load the commit data) so that it made the navigation to a specific file/folder even faster.
Also, they would need to store that, and I don't necessarily want my browser storing my entire directory structure (and possibly file contents) for any amount of time other than when I'm actually browsing it.
Try it out, it really is very fast without having to eagerly load anything at all.
That would be more work for the GitHub guys, but I'm pretty sure it would work.
I'm just pointing out that since they've changed the arrangement (e.g. no page reload when navigating between directories), they could take advantage of it to get a huge performance gain rather than a small performance gain.
Try jQuery Mobile repo:
https://github.com/jquery/jquery-mobile
[Obviously, you will need Chrome/Firefox 4/Safari.]