Bob,
Check out some ideas here. http://www.marketingrockstarguides.com/how-to-run-ab-email-tests-in-marketo-505/
There is an article on this exact question. It won't be 100% automatic though:
Setup the A and B emails.
Setup the smart lists (or do Random Sample)
Smart List: Your Audience
Flow:
Send Email A if Random Sample is 5%
Send Email B if Random Sample is 5%
Do nothing
Wait 4 days
If Not Was Sent Email (Loser) then Send Email (Winner) (which will go to rest of list.
So you need to make sure you check the email performance tab of the campaign and then select the winning email for the last flow step.