Is there an API set for Ubuntu One? For example, an API to sync contacts, bookmarks, etc.
The common way to store something persistently and have it synchronise with Ubuntu One is to just use desktopcouch. Here's an example:
Where "example.com" should ideally point to a description of your record format.
And to retrieve the information afterwards:
The API to desktopcouch is pretty straightforward, have a look at the Desktopcouch Documentation to learn more about it.
What @Stefano said. Also, right now there isn't a single place where you can go and read up on all the different APIs related to Ubuntu One, but we're building it in time for Natty.