Changing margin reqs were an attempt to limit the rise in PMs - but not in the way that the conspiracy theorists would suggest.

Commodities are traded on very low margins on the commodity exchanges - you're used to 50% margin? These guys work on 5% or less.

Problem is, when the intraday volatility of a commodity is greater than the margin req, that's a recipe for disaster. You get a crazy bubble form as people realize they can make >100% gains in one day on, e.g., silver; but then, if silver ever goes down by 5%, all the investors get wiped out. Completely.

This will spill over into other markets, like equities: for example, if someone's playing physical silver contracts, his house might also have a strong position in FVI, a good silver miner. When he gets the margin call, not only does he have to sell all his physical silver, but he might be forced to dump a pile of FVI too, to satisfy the margin call (in fact, that's possibly what happened on Tuesday, when FVI started the day at $4.50 only to bounce off $3.80 by the end of the carnage).

Nobody wants that sort of extreme risk, especially not spilling over into other markets! So the commodity exchanges have to periodically adjust the margin reqs for each commodity to ensure that the margin is always greater than the intraday volatility of the stock.

Silver's margin will have to be upped again, unless this month's silver bubble proves to have been popped by the magin upping.

You'll see margin upped on a whole pile of other commodities soon as well - basically, anything that's been skyrocketing in price recently.