I know it's probably well down your list right now, but a simple switch to show the correct currency sign for those of us not in the US would be nice. I personally just stick in the Euro value of my fuel, it makes it easier for me to track fuel usage independant of currency fluctuations.
Yep, it's a bit down the list at the moment but we definitely plan to offer a handful of currencies. We're storing all data in US units and converting where necessary, but obviously the system is expecting US dollars as the currency right now. From a data-sharing view we'd prefer that you entered the value in US dollars, but I realize that's unrealistic. Once we have multiple-currency support we'll probably include a way to convert existing entries to the currency you used when entering a fuel-up if you didn't intend US dollars.
On the subject of other currencies, I just posted this in the MeFi thread but I should repeat it here: in the UK the petrol prices are in pence rather than pounds, since they are almost always xyz.9 pence per litre, e.g. 112.9 pence per litre. This is, I assume, to avoid ugly numbers like "£1.12.9".