Trip Cubby (App Store link) is a mileage log on steroids. It has a well thought out interface giving you the flexibility to put in as much detail as you need about trips. When you launch the $9.99 app, you're greeted with a simple list of trips and the option to add a new trip. It's then that the power of this app shows up, since the amount of information you can capture about a trip is very complete and customizable.
If you need to supply a trip log to your accountant, Trip Cubby not only creates a comma-delimited (CSV) file, but opens an email view so you can send it to someone. Developer David Barnard notes that they're working on an online backup system for Trip Cubby and other apps by App Cubby so your iTunes backup doesn't "lose" your info. Check out the gallery for more screenshots of Trip Cubby.