Kevin Feldman, a Managing Director in the iShares unit of BlackRock, stated this in an email, refering to the SLV ETF
'Although market makers will generally take advantage of differences between the NAV and the trading price of Silver Trust shares through arbitrage opportunities, there is no guarantee that they will do so'.
I would not call arbitrage hedge funds 'market makers' as such, as I said I would assume that a fund associated with black rock would be the dominant market maker for the SLV, but hey, I could have it completely wrong.
cheers grant
the above is opinion only and written without prejudice