(Well, not a specific floor, but a conceptual one: the self-interest of strangers.)
Since the economy seems resistant to participate in our ongoing attempt at a 2003-replay, the primary driver I can see of continued growth in the market is weakness in the USD. Open question: is our Federal Reserve, and national government, willing to allow the USD to set new record lows (<72 vs world currencies) in order to "stabilize" the system? If not, what will be the signal to stop?
Currencies aren't necessarily all that different from stocks. Currencies are claims on the performance of a nation's economy and its assets. Currency value stems from investor desire to purchase items in that country, whether the items are financial assets (stocks, corporate bonds), real assets (manufactured goods, services, real estate, commodities), or incentives for government assets (eg, high interest rates in gov't securities that can only be purchased with the currency).
Who are the shareholders of the USD? US citizens, and countries with foreign treasury reserves.
What does a conference call with investors in the USD sound like? No idea what they say behind closed doors, but the closest thing to a public transcript is probably comments from a G-20 summit. Unlike stocks, I don't think there is the concept of Reg FD; that is, if a foreign government tells the US government that the deficit is much too large, I don't think there is a requirement to share that with us retail holders in the general public at the same time.
We can mostly guess at the actual pressure being levied. Hopefully the private conversations are much more colorful than the public statements. A sample sign of what might be said behind closed doors is alluded to in this comment from Paul Volcker:
Reuters: Answering a question from the audience, Volcker said he
doubted China would embark on a major reduction of its U.S.
dollar-denominated assets, partly because the alternatives to
the dollar, including the euro and the Japanese yen, do not
look that great either. China has "a great interest in the stability of the
dollar," he said.
Bloomberg has the audio.