Like many others I had problems making my files visible on Ubuntu 12.04. In the end I got all I needed from nexus7tablethelp.com but had to use the tip in the comments from Daniel to make it work better.  This does not give me access to “sdcard” on the nexus 7 so I am still stuck downloading anything like backups.

I’m assuming that by Ubuntu 12.10 this functionality will be native, but at least I have some access now.