Maybe…
I noticed today that the change log and version file have been commited to the SVN branch. This has tagged the release to be made pretty soon.
Well, I’m only guessing but I don’t see any reason to be wrong 
So Watch this space – and joomla.org for the official announcement. Remember you should upgrade to Joomla 1.5.8 as soon as its available to ensure your security and stability.
[...] night, as expected, Joomla!® 1.5.8 was released by the Joomla! Project on the official [...]