I use two-step authentication for both my GMail and Google Apps accounts as well as dropbox.
Up to now I've been using Google's iOS authenticator app - but this app has issues in iOS7
The first one is of course the most serious.
After some googling I found that there are other TOTP app's out there. I settled on Duo Mobile
After following the third-party accounts page I had both my google accounts and my dropbox account in the app and it's working nicely.