One, there isnt enough property owned by the us government to float a currency on.
Two, it would be awfully easy to infringe on private property rights if you based it on total land in the US.
Three, you couldnt convert the currency to land without their being major problems, like China now owns wyoming and the dakotas type problems.

One and two I don't see as being a problem for some folks in the govt. (Not saying it will happen, just that I could see it happening with the right sequence of events, like Germany in 1923. I think Correia had an essay along those lines...)

Three... well, three could be a sticky point, unless war is declared by China on us. Voids all debts to the aggressor. (Again, not saying that will happen, but not impossible)