If you upload a candidate package, and then click on "maintain", then you get a page that says "Here, you can delete a candidate, publish it, upload a new one, and edit the maintainer group.". However, it isn't actually possible to do any of those things.
Some more information:
Before going live, we should either finish the functionality, or disable it completely (i.e. remove the "maintain" link, and perhaps rename "upload candidate" to "check package").
http://www.haskell.org/pipermail/cabal-devel/2012-September/009111.html says that this is almost implemented, just the relevant links missing.
We (mainly @edsko) have done quite a bit of refactoring to make the candidate feature more practical, so other features can be instantiated both for the main and candidate packages. Need to check latest status to see what's left to do...
This is not a Cabal issue.