I did a search and back in winter of 2008 there was some discussion of a FB badge and/or app as well as an iPhone app - I'd really love to see these implemented and they would give Fuelly a much greater exposure. I can see a nicely designed app getting Apple commercial airtime considering Fuelly's appeal.
No, we're not working on either of these. We're just two guys running Fuelly as a side project so we can't tackle everything we'd like to do. We make no money with Fuelly, it's just something we wanted to see exist in the world. So it's tough to justify development cycles for it when we have work to do on our other sites over at MetaFilter.
That said, we have Fuelly Mobile--a version of the site optimized for the iPhone. I'm not sure what an iPhone App would provide that we don't already have. It'd be great to see Fuelly in an Apple commercial, but is that potential enough to justify weeks of development? And more importantly, are the improvements that an iPhone app would provide to the fraction of Fuelly users with iPhones enough to justify the time involved?
I also left a comment a while ago explaining how to import Fuelly fuel-ups into Facebook. Writing a Facebook App would take a few weeks too, and it'd end up working in about the same way as the existing import feature, so I'm not sure what advantage there is to jumping through all of the hoops to create an official app. I know Facebook Apps are hot right now, but I'm just not sure it's worth the cost in time.