I've got Dropbox installed on both my Windows 7 machine and my Ubuntu machine. I've got "Enable LAN sync" ticked on both.
However it doesn't seem to be syncing via the LAN. Is it possible to sync across a LAN across different OSes with Dropbox?
Yes its possible of course but Dropbox behaves stupid sometimes. Try out the experimental build, Ubuntu needs that one I guess. Just unpack it and start the dropbox file.
Here is the official forum thread for experimental builds.