Easy:

Use DropBox, Carbonite, Amazon, Office365, or other such cloud services.  The file is encrypted locally by the program itself, and most services encrypt the file anyways before it goes up to their servers.  In fact, if you put all three files (the dll, the exe, and the dat file) into the cloud, then you can get to your e-mail so long as you remember your [insert cloud service] password AND your GAC password (which are different, riiiiight??!).  This way, aside from the 10 emergency codes that Google gives you (you wrote those down somewhere, riiiight?!?), you can still get to your mail in most disastrous situation.

I say most... you still need a Windows Vista/7/XP box with internet and .NET 4.0, of course.

USB DRIVES, BAD!

I've found that using removable media with the Accounts.dat file to be a BAD move, it gets corrupted way too easily.  Maybe I need to put more file-buffer-flushing routines in my code, but just don't put that file on a flash drive for now.