Dropbox is not open source at all. There is a very thin open source GUI that is open source, but when you install it, it goes ahead and downloads 37 MB of binary files. These binary files include their main closed-source daemon, ~/..dropbox-dist/dropboxd, as well as lots of shared object binaries from *mostly* liberally-licensed open source projects. The DropBox people have no intention of making that code open source, and their communications protocol is also closed.

FWIW, there are some concerns that the binary closed-source code is breaking various open source licenses, due for example to their inclusion of rsync and readline binaries in their binary package.