I'd love to see support to send updates via text message (for those of us without data plans)
Programmatically, this wouldn't be terribly hard either. Even if you entered a text message with something as cryptic as 125230 2.79 11.20, the software could confidently guess that 125230 is the odometer, and that we paid 2.79 for 11.2 gallons of fuel.
If users wanted to be more specific, they could easily add $, gal, and mi to their numbers.
Most phones also now support sending text messages to email addresses, which would make the implementation even easier!
Except schmod, we allow users to have multiple cars.
So in effect you'd have to send a text like: "hondacivic 323.3 $2.399 14.301" and remembering both the title of your car plus the order of entry, plus the proper numerals and decimals, it starts getting too complicated for a simple text.
I've found even though I have an iPhone, I'm usually in a hurry so I just jot down the odometer mileage on my gas receipts with a pen in my glove compartment, then about once a month I sit down and log them to the site.
It's now 5 years later and I'd really love that API now :-) This post came up first in Google, so I'm guessing there still isn't one... I like the mobile version a lot. It's fast and super simple, but you can only ADD fuel ups, not see previous, or statistics.