When you save the state, it will create a snapshot file to:
[VirtualBox Directory]/[VM Name]/Snapshots/*.sav
Which has a time index; the latest time index should be read first.
For example I just made one, it's stored on my machine:
~/VirtualBox VMs/Arch Linux/Snapshots/2013-05-08T11-39-00-247704000Z.sav
You can either bring these with you on a USB, and put them into the snapshot folder on your host; or use a service like Dropbox to automatically sync them, using soft links:
$ ln -s <LINK> <TARGET>
or in Windows:
> mklink /d <LINK> <TARGET>
or for a directory junction
> mklink /j <LINK> <TARGET>
Where <LINK> would be in your dropbox folder, and <TARGET> would be the directory for Snapshots.
You would have to do this on both machines.
Depends on how comfortable you are in Command Line.
Hope that helps.
If it doesn't, I hope it guides you to the right path.