I got carrier file 5.6 as well. As of October 15th, 5.6 is the latest carrier file, not 5.5. It was posted on this site's blog: http://www.theiphoneblog.com/2009/10/14/mystery-solved-att-carrier-file-update-56-iphone/
Is 6 even out? I'm running 5.6, and it says I'm running the latest software.