Given that this is a new project, why focus on python 2.7, rather than 3.4?
1. 2.7 is a stable target. There will be no further features added to the 2.x series.
2. 2.7 is a known target. Other experiments like PyPy have already tried many paths to improve 2.7, so what doesn't work is reasonably well known.
3. 2.7 is a widely deployed target. This is less important from an "adoption" point of view, more so that there is a lot of software written that targets 2.x, meaning there's more software to test with.
4. Finally, I suspect much of Dropbox's codebase is 2.7[1], and since this is an effort to improve things for Dropbox, it makes sense to spend the initial efforts there :)
[1] checking the version of Python bundled with the Dropbox client confirms that it, at least, is 2.7
/Applications/Dropbox.app/Contents/MacOS/python --version
Python 2.7.3