Who would put an expiration date on their currency?

From Ethan Zuckerman's blog:

Zimbabwe is desperately short of foreign currency, and their own currency is in an inflationary free fall - CNN reports an annual inflation rate of 1200%. For businesses like my hotel, which makes a lot of purchases abroad to feed guests and keep us in clean sheets, it’s critical to have dollars on hand, since few vendors will accept payment in Zimbabwean dollars. And it’s hard to buy dollars, and probably impossible to buy them at the official exchange rates. So the hotel needs me to pay them in dollars so they can buy the goods to run the hotel for the guests who pay in a currency that’s losing its value every day.