Unfortunately, the web interface for U1-synced notes was removed on February 28, 2012. Note sync will still work, but you won't be able to access them online.
U1 Notes web interface hasn’t really been working as well as we want it to. We have looked at the improvements required and the resources needed to raise the overall quality of the notes web interface. However, with everything that we have to work on in 2012, plus continuing to improve our core services, we just won’t be able to give it the attention it needs.
For those of you syncing notes with Ubuntu One, this means you’ll continue to sync them via Tomboy just as you do today. The only difference is that you won’t be able to access these notes via the Ubuntu One web site after February 27th.
The Ubuntu One Blog