Quote from dearinfinity:
I see what you are saying but isn't the salient NAV a way of insuring against this? People will know the underlying value of the ETF, and so long as volume is moderate, will it not be maintained? I don't understand how price decay can occur so long as the NAV is public.
check out this post and take a look at the attached spreadsheet
http://www.elitetrader.com/vb/showthread.php?s=&postid=2372699#post2372699
Hopefully that will make it easier to understand.