When I'm going into a client's office to spend the day working with them, one of the first things I do is spend 10-15 minutes perusing the commit history to see what's changed in the 4-5 days since I last looked at their codebase. I'd love to be able to do that, as well as do code reviews, on the bus/subway on the way into the office.
That would probably be bloody hard though, making code readable on a mobile device. Also of limited use, if you can't edit it.
I could see it useful to e.g. read & review pull requests on the move, but there really isn't much you can do with just a phone's surface.
And if you have accessories, why not carry a small laptop you can tether to your phone?
aGit currently does not support push and commit, but I believe that is in the works.
I've used this setup on my Transformer Prime. My G2 phone is, of course, smaller, but I have used that to shell into my server and edit config files to fix a problem, so reading and editing code on a mobile device is certainly doable.
A 7" screen device such as the Nexus 7 might a nice balance though I have physical keyboards for my phone and tablet, which I really like.
I have done some editing using the on-screen keyboard on the TF201, not the end of the world, just not as nice as a more conventional keyboard. The constant toggling to get to special keys is tedious.
I know the Asus Transformer I have is fairly niche for a tablet and thus few developers are likely to target for its feature-set, but it would be fantastic at reading or even writing code, with proper app support.