How can I view a list of email addresses in a given segment?

You can lookup specific email addresses in the test window, and can also preview the contents of emails for them:

However, there isn't a way currently to view the complete list of subscribers in a given segment all at once.  

There is a bit of a hack that you can use as a temporary workaround.  By adding /pending-recipients to the message URL, you'll see a JSON response showing those email addresses:

We'll be rolling this into the actual app interface soon but this is a temporary workaround.