Along with the creator preview video just out today, Linden Lab is releasing some interesting info about the Sansar Dollar, which operates very much like Linden Dollars, in the sense that their value is pinned to the US dollar, but fluctuates according to market demand. Unless I missed another example (and this is certainly the most prominent one), this is the very first virtual currency with real money value in the post-Oculus era of VR.
However, ending speculation among many SLers, no, it will not be immediately possible to buy Sansar Dollars with Linden Dollars, or vice versa:
Linden Lab: The exchange rate between USD and Sansar dollars will be floating and will vary according to supply and demand. The rate will not necessarily be equal to the exchange rate between USD and Linden dollars in Second Life (which is also floating).
To begin with, the exchange rate is about USD1 to S$100.
Road to VR: Can owners of Second Life currency transfer it to Sansar for use there?
Linden Lab: For now at least, not directly. At this time, Second Life users can sell Linden dollars for USD on the LindeX, and then can use those USD to purchase Sansar dollars.
In the future, we could potentially allow Second Life users to buy and sell Sansar dollars for Linden dollars, but because the two platforms will have different business models, fee structures, and exchange rates, we’d want to be careful not to set up a system that could be exploited.
That's very wise, and when it's spelled out that way, it makes me think there never will be an active exchange market between Linden Dollars and Sansar Dollars. Some kind of controlled "buyback" campaign, where users can exchange a set block of Sansar Dollars directly from Linden Lab for a set price of Linden Dollars, is probably the better way to go.
Speaking of which, Tom's Hardware raises a good question about the 500 people who are already in Sansar's early access program: