Sync Tomboy through Ubuntu One and local folders
Solution 1:
Have a look at inoticoming in the repositories. With this, you should be able to sync to a local directory, and have inoticoming initiate a copy of the files to somewhere else (DropBox, Ubuntu One) as the changes come in. This will also allow you to keep a copy of the previous version somewhere else as well, as I think it lets you perform an operation on the existing files using one of its options.