Joomla 1.5, 2.5 and Joomla 3 are all affected by this error and being contacted by Google with a message saying:
This error is caused by the robots.txt file in Joomla installations, which blocks access to the Google Bot to certain key files.
We can't simply give googlebot access to all files in Joomla, as this would cause Joomla system files to be displayed in Google search results - which if nothing else would be bad for SEO.
The solution is to add the following to the top of your robots.txt file in the main file directory of your Joomla installation:
In addition, if you are using Joomla 1.5 or 2.5, I'd suggest removing the line in robots.txt that says "Disallow: /images/"
It is generally good for SEO to enable images to be indexed by Google bots.