I'd look at this considering the entire stack of software involved
1) Rails - pretty much doesn't care what you do in the public directory, rails assumes the Public directory is there more for Apache or whatever web server you are using.
2) Apache or whatever, - almost the same answer for the Rails layer. You might want to peruse for Apache plugins that address the needs of very large filesystems, but since that is pretty in much the realm of the OS involved, that's where you want to be concerned.
3) The OS. Now if it were me, and I knew the Public directory could get extremely large, I'd deploy the app at Joyent, because they use Solaris, and Solaris has the XFS filesystem, and Solaris is more stable and solid than most of the Linux out there. I'm not sure, but I believe Joyent can arrange for some special hardware and mount it up as XFS, get it backed up for you, etc.
Joe got a job, on the day shift, at the Utility Muffin Research Kitchen, arrogantly twisting the sterile canvas snout of a fully charged icing anointment utensil.