(Note: I haven't done any mobile development for a while, and certainly none related to Ethereum. The below answer therefore includes a level of conjecture. I don't know of any existing efforts.)

Android apps are (most often) written in Java, so it's plausible that the relevant parts of EthereumJ could be ported with relatively little pain.

It's also been possible to write Android apps in Go since (Go) version 1.4, though I'm unsure how well supported this is. It does, however, raise the possibility of a Geth port, or partial port. Similarly, looking at Go's available mobile packages, it looks like iOS support is also there.

(Note: I haven't done any mobile development for a while, and certainly none related to Ethereum. The below answer therefore includes a level of conjecture. I don't know of any existing efforts.)

Android apps are (most often) written in Java, so it's plausible that the relevant parts of EthereumJ could be ported with relatively little pain.

It's also been possible to write Android apps in Go since (Go) version 1.4, though I'm unsure how well supported this is. It does, however, raise the possibility of a Geth port, or partial port. Similarly, looking at Go's available mobile packages, it looks like iOS support is also there.

Edit:

Or I could have just Googled... (Thanks Ellis for the prod.)

This entry on the official GitHub repo describes how to perform an Android-Geth port, and references the more general Ethereum cross compilation page. From there, the xgo README has some further details of building mobile libraries.

I can't immediately see any pre-built libraries/images.