When I first connected my Nexus 7 to my laptop running Xubuntu 12.04 I was shocked that it didn't appear as a USB device as my Kindle Fire and DroidX do, but after some research i saw they changed how ICS and JB use USB connectivity.  Now it uses MTP or PTP where PTP is more for pics and MTP is for accessing the file system.  Problem though is Linux doesn't natively support MTP.

So here's how to set it up:

Set up a UDEV rule via the terminal by opening the rules file

sudo nano -w /etc/udev/rules.d/51-android.rules
And entering the following line at the end of the file (be sure to use your user name at the end!):

SUBSYSTEM=="usb", ATTR{idVendor}=="04e8", ATTR{idProduct}=="6860", MODE="0600", OWNER="<your-username>"
Now install the tools and set a mount point by entering the following commands one line at a time

sudo apt-get install mtpfs
sudo mkdir /media/GNexus
sudo chmod 775 /media/GNexus
Plug your Galaxy Nexus in, and enter the next line in the same terminal window:

sudo mtpfs -o allow_other /media/GNexus
Drag and drop through Nautilus, and when you're done and need to unmount, enter the following:

sudo umount mtpfs
[Source]

As for OSX it's also not supported natively, but Google has this app to add support - http://www.android.com/filetransfer/ . 

I haven't found any way to enable UMS (USB Mass Storage)  on the Nexus 7, seems Jellybean only supports MTP or PTP which is less than ideal.  Also it stinks that Linux users have to go through this hoop given my Kindle Fire with UMS support would just show-up as a mounted drive automatically when connected with no hassle. 

I'm sure someone will address this soon, but I'm betting it won't be from Google since this has apparently been an issue on many devices since ICS.  I found a tutorial where someone released a patch for the Samsung Galaxy S3, but I'm not sure if this'll work with the N7.  Hopefully a similar one will come out soon though for us.

I  came across this, especailly good for Unity users. http://forum.xda-developers.com/showthread.php?t=1773598

Something to note, this weekend when I turned on my laptop it hung after the Xubuntu slash screen, and dmesg showed the rule I added in /etc/udev/rules.d/51-android.rules was the culprit. I removed this, rebooted, and all was good.  So at least on Xubuntu 12.04 running XFCE 4.10 this might give you some unexpected results.  I'll do some testing and report back on what I find, but just an FYI for any other Xubuntu users out there.

Also something to note, Xubuntu 12.04 comes with XFCE 4.08, but I upgraded to 4.10 when it was released... and it's not fully supported on Xubuntu 12.04.  That might be the problem, but I won't know for sure until I get into it more.

just an update--found that i can talk to my 7 from linux using ADB. i did it the hard way before i found this link which will put the tools in you software sources.

              http://www.webupd8.org/2012/08/install-adb-and-fastboot-android-tools.html

  unfortunatly or fortunatly from which ever prospective---command line only   
plus once installed does not require root in a terminal. to many commands to mention but

adb start-server
adb pull /sdcard/??folder/ (will pull files to home directory)
adb kill-server