Personally I don't think they will ever stabilize like physical currencies until (if) they become as widely used.
The point is physical currencies are not stable against another one, this is why Bitcoin cannot be stable as well
The point is, that physical currency fluctuations are in tenth of a per cent range daily, and not like Bitcoin everyday fluctuations from 10-25%. So if you compare, then i would say that physical currency are almost stable against another one.
On Forex, with a trading volume of 200-300k USD in your hand you can make a bunch of 1k bucks per day, with luck. If you take the same amount $$ and trade on BTC, you can make easily 20000-30000USD per day (or lose of course). And the demand is not even tenth of a per cent daily.
In BTC you would affect the market price if you trade with 200-300k bucks daily...
