While in the "departure lounge" at LAX I made two purchaces within an hour:
NORTH DELI BAR-LAX LOS ANGELES CA 7.60
13.78 USD EXCHANGE RATE 1.813158
DFS USA - LAX TOM BRAD LOS ANGELES CA 11.58
21.00 USD EXCHANGE RATE 1.813472
They appear to be running the transactions through live, and calculating the exchange rate to (maybe) the second. The complexity implied by the latter surprises me - I wouldn't have thought that they would be that exposed to exchange rate fluctuations that they couldn't instead batch the rate daily.
The other fun part:
If you make the minimum payment of £10.00
... your estimated interest payment next month is £11.33.
Cool. Not only won't I ever pay the debt off, it will actually get bigger.