Now have custom URLs for sites created using Google Sites on your domain

Now Google Apps domain administrators can map public sites hosted on Google Sites to their own domains. Sites can be mapped individually to a specific URL, like http://www.example.com, and as a group under a specific URL, like http://sites.example.com/[name of site].

So in your company if you have a site each for HR, Marketing, etc you can now have URLs like:

http://hr.example.com
http://marketing.example.com

and these URL will be diverted to site created using Google Sites. To do this you will need to set these URLs from admin control panel. The option is available in service settings in for sites - "web address".

Read more on Google blog
Google Sites Blog: Google Sites on your domain