The standard is L, R, C, LFE, SL, SR. This order is used in Dolby Digital and DTS.
Unfortunately there seems to be more than one standard. See this article. I tried to find something difinitive on the Dolby site and my search skills seemed to have failed me. All I could find was this Dolby 5.1 FAQ http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/42_DDFAQ.pdf
While trying to find the "right" answer for this question, I ran across this page. Kind of cool. I imagine that video people could do a better job visually with this. Might be a fun contest one of these days.