well - I'd thought 5.1 "rears" could go on back wall.
please see bottom illustration at : http://www.polkaudio.com/education/tech_article.php?id=27

Also, this site : http://www.technosound.co.uk/nav.php?pageid=hcg_settingup mentions: "With the seating against the rear wall, bipolar speakers must be placed on the rear wall half way between the seating & the side walls"

I'm hesitate to mount things to side wall because they would be so far away; room is 22 feet wide - and one of the side walls has our fireplace, so my wife would veto it...

Thanks for the feedback so far!