Turns out this answer was mistaken, see Andrew's post for the correct answer.
Yes I does.
I've tested it with a standard free dropbox account with the dropbox folder in a btrfs volume.
The test was:
ln /media/username/volume/Documents/testfile /media/username/volume/Dropbox/testfile
ls -li /media/username/volume/Documents/testfile
ls -li /media/username/volume/Dropbox/testfile
The first number from the
ls -li command shows th inode number, the number between the permissions and the username is the link counter (shows how many hard links the file has).
Immediately after running the ln command the dropbox try icon showed activity, and effectively uploaded the test file.
I've also tested to make a second hard link inside the Dropbox folder and it also uploaded the file, I'm not sure if it's aware that's only one.
My test file is a small 18KB .ods file, but it should also work larger files.
About hard links and inodes:
When you create a hard link of a file you are giving it just a second name in a different folder (or the same if you like to).
This is files are not really placed in folders they are on the hard drive identified by an i-node (depends on the file-system type).
So what are folders?
Folder are just lists also identified by an i-node that contain the names of the files and other folders that are 'inside' of them. Each element in this list point to it's corresponding i-node.
So if you create hard links both/all point to the same i-node and therefore all of them are just files (the same one).
Be aware that some apps refuse to do their job with files that have hard links. Other apps may accidentally or intentionally break hard links turning one file with say 2 names into two files with one name each.
Wikipedia gives us a more info:
To create hard links you can use
ln which is a command lines tool, You can use it like this:
ln TARGET LINK_NAME
Where target is the existing file and link_name the new name of the file.
When you are done you can check with
ls -li the i-node number of the file and see that both links have the same number.
Here is the man page: http://linux.die.net/man/1/ln