Its a 0-day SRU (Stable Release Update). In simpler language:
1) An SRU is an updated version of a package which is uploaded for a release which has been released already. SRUs are more commonly known as 'software updates' and these are the things which come up in Update Manager.
2) 0-day SRU means that it is an update which will land very soon after the release of Ubuntu 11.04 (usually the same day as the release itself).
According to the SRU procedure, every SRU package should be first uploaded to the proposed repository. Then, the bugs fixed by it should be confirmed by a user as being fixed before the package could be moved to the main updates repository.
In this case, the Unity 3.8.12 package fixed 5 bugs, 3 of which have been verified (as of the time of posting this answer) and 2 are still in need of verification. The Unity 3.8.12 package is lying in the proposed repository, and will stay there until someone verifies the 2 remaining bugs. Once that is verified, then you'll get Unity 3.8.12 as an update from Update Manager. This should happen in a maximum of 4-5 days from now.
You can check the status of the Unity package in the proposed repository at this link. Browse down until you reach
natty and then look at the Unity package row. Under
changelog bugs, a blue bug link is a bug which is awaiting verification, a green bug link is a bug which has been verified successfully by a user, and a red bug link is a bug which wasn't verified successfully (in other words, the bug fix didn't work).
If you are impatient and want the package right now, then you can enable the proposed repository in your system by following this guide (WARNING: not recommended as the updates could be untested and might lead to regressions) and then you will get the Unity upgrade automatically. Once you've upgraded, then you can yourself verify a bugfix and go to the relevant bug page and comment that you've successfully verified the SRU. Also, tag the bug with
verification-done and remove the
verification-needed tag if you have verified the SRU successfully. If the bugfix doesn't work, then tag it with