I think I figured it out this morning....
Activate the new 2013 flavor of the Opportunity form under settings and publish. When you use that new form, scroll down to the to Product Line Items section. Price List and Revenue are two fields listed above the line item details. Pick a price list
and change the value of Revenue to System Calculated. Now when you add an opportunity product the extended amount will calculate.
I'm in the process of merging my legacy 2011 forms into a copy of the new 2013 Opportunity form and expect that will fixed the issue.