I'm also interested in this. Cloud storage/syncing/sharing/backup like Dropbox is now an essential part of my workflow (and for sharing stuff with my GF too).
I've been planning to switch my Linux box to PC-BSD (already dl'ed 8.1) and just hit this issue.
From looking at the Dropbox forums, although various partial hacks (like enabling Linux ABI) are mentioned, it just doesn't work right on FreeBSD
(e.g. Dropbox will download but not upload). Dropbox on Linux is dependent on a file system service called inotify, which apparently is planned for FreeBSD sometime in the future.
I don't know yet about Spideroak or Zumodrive compatibility with FreeBSD.
An open-source "Dropbox alternative" called Sparkleshare is in development... but it's far from finished and no info about it working on FreeBSD (beta works on Linux/Gnome/Nautilus; Windows and Mac versions "coming soon"). Also the online part of it works with completely public storage solutions: not great for storing materials for my clients or for personal stuff.
When I first thought of moving from Linux to PC-BSD, I thought "Well, 99% of the apps I use on Linux are FLOSS, so they'll be available on *BSD too". It's only just now that it hit me that 1% per cent that isn't FLOSS (Dropbox) is crucial to me and that it doesn't work on *BSD. This could actually be a deal-breaker for me, keeping me on Linux and off PC-BSD.
I hope someone can come up with something like Dropbox et al that works on FreeBSD.
Discussion re FreeBSD support on Dropbox forum