A friend shared a folder to me, but, when i have accepted the sharing, i can see the folder and the files, but ubuntu one indicate the folder are shared from "Unknown user". My friend have setted up the name correctly in his profile.
This is a known issue. Sadly, there is no (current) way to workaround this problem: your friend's username was originally created with an empty display name in SSO (see bug #709496 in launchpad for details), and Ubuntu One stores user data for shares only at user creation time. So, even if your friend set up his display name in his profile, our database to build shares will no get updated. This last part needs to stay this way as long as we support Ubuntu One clients from Lucid, which do not support that the name of the sharing user changes once the share is created.
Bug #709496 is now fixed in the SSO servers, so the SSO desktop client app in Ubuntu 11.04 is already requesting to the newly created user to set a non-empty display name. For Maverick systems, we're trying to have this SRU accepted and released soon: bug #742678.
I'm sorry I can't give you a workaround for this. In the next cycle, we're planning on improving the Shares experience. Thanks, Natalia.