I don't know if Gold and Silver are in a "bubble" - overall market particpation is still low - its not something "everybody" is doing- at least not yet. Even in good times, many kept 10% of their total wealth in gold and silver as a hedge. However, there is a lot of paper gold and silver being issued and from that data I have seen the COMEX has about a 10:1 ratio of gold on paper vs. gold on delivery. Like any old fashioned bank the COMEX could see a run, and collapse.. ironically where most pre FDR ncontract had a Gold clause, most Comex contracts have a fiat clause - where they agree to pay X in dollar if physical delivery is not possible... AND this has been happening for large orders.
The underlying fundamentals are an unstable fiscal environment - worldwide. If that changes, then Gold/silver will drop. However, the method by which that changes are important. Other countries are actively looking for a way out of the dollar as a reserve currency. BRICS mad deals to use thier own currencies, OPEC has threatened to price oil in something other than Dollars and the world organizations are pushing SDRs as the new currency, which depending on what plan you look at, calls for potential backing of those SDRs with a basket of Commodities - including Gold and Silver.
I have to admit in this climate I am only looking to preserve my wealth, and I am not "investing" in grow it. If there is a sudden fiscal crisis then Gold and Silver will really bubble - and that is the time to get out if you are "investing" in this way (what is the difference between investment and speculation?) - but you would need to quickly convert your Gold/Silver into tangible assests- as converting them into $$ would be pointless. As will be converting them into outher currencies. When the dollar goes, so does 1/4 of the worlds consumer market - and this is going to hurt EVERYONE. Another currency will be better, but then you will also be at the mercy of foreign nationals who will be none too sympathetic towards the Americans after a sudden default.
For those who think a dollar default is impossible, I suggest reading "this time is different" - its a history of monetary defaults. Unless something really amazing and miraculous happens in congress, default is GOING TO HAPPEN. All that is left is the form of the destroyer- an outright default - or a default by inflation. The latter course is pretty much locked. The Fed is going to end QE2 soon, and without China buying - our rates will go up - and then because most of the national debt is in short term bonds, we will be in effect forced to refinance at ever high and higher rates ( this is how the Clinton "surpluses" we created - they moved long term debt into short to get a lower rate..)
Greater interest rate payments will simply make the problem worse until QE3 is started to pay the interest, or the government defaults by offering a new "payment plan"-- if its death by interest you will see periodic pullbacks in gold and silver, but you will NOT see a reverse in trend unless a new world currency ( not based in part on gold/silver) is adopted as a new reserve, or the US actually does what it required to reign in its spending.