No immediate plans, but it is something we'd like to do sometime. Sorry I can't be more specific than that. Fuelly is a side project for me and Matt, and if we had infinite time we'd love to build an API for Fuelly. As it is now we spend most of our time on MetaFilter.com which pays our bills and is a much larger operation to manage. We operate Fuelly at a loss, and we hope someday it can become self-sustaining.
In the meantime, some Android developer out there could probably come up with an interesting way to integrate with Fuelly using our existing import/export features. It could be as simple as offering an export of collected data in Fuelly's format.
If you search existing Android apps you'll probably find several to track mileage. Don't let the lack of a specific Fuelly app keep you from tracking your mileage however is most convenient for you. I know there are at least half a dozen mileage tracking apps for the iPhone so we're not the only ones with this idea.