I think that the problem comes from sunbird.
You have a version karmic and there is a rule "Replaces: libnss3" which holds back libnss3. Because libnss3-1d is renamed to libnss3 during the upgrade to Natty the upgrade fails.
If you remove sunbird and its dependencies you should be able to upgrade.
Note that sunbird has been removed from the archive since Lucid and is available as Thunderbird Lightning extension.
I think that the problem comes from sunbird.
You have a version karmic and there is a rule "Replaces: libnss3" which holds back libnss3. Because libnss3-1d is renamed to libnss3 during the upgrade to Natty the upgrade fails.
If you remove sunbird and its dependencies you should be able to upgrade.
Note that sunbird has been removed from the archive since Lucid and is available as Thunderbird Lightning extension.