No, like we mention in the FAQ we don't have an iPhone app. We're a very small team with limited resources so we feel like it's better to put our energy into Fuelly Mobile that can run across dozens of types of mobile phones instead of creating native apps for each of them.
You might try adding a fuel-up via SMS. I've found that's the fastest way for me.
I've just saved the Fuelly mobile site as a bookmark. I don't know if you have done this before but the bookmark will show on your homepage almost like an app. One click takes me to the Fuelly mobile site, a second click on my vehicle and I'm ready to input the data quite easily. As much as I'd love an iPhone app, until there are more developed features of the site I'm just not sure it's needed. I'm actually typing this from my iPhone!