To Bob's ease-of-use comments, I too would like to see this functionality available directly from `gradle`. Whether it's using some variant of Baruch's approach, XSLT, or the forthcoming embedded Maven support, the point is that the 'how' should be an implementation detail to the user, who simply expresses through a convenient CLI 'what' he or she wants to do. In this case, the 'what' is converting an existing (set of) pom(s) to Gradle script(s).
I say when in doubt, ask yourself "what would Git do"? Git would certainly integrate this as a subcommand, something like `gradle convert`. Because doing such a conversion could be arbitrarily complex with many options, Git would also likely provide an interactive mode (`gradle convert -i`).
I think Git is a shining example of command-line power and usability. We should strive to make Gradle's CLI just as inclusive and even more user friendly. Gradle is about more than 'building your code'. The vision is that Gradle is your one stop for project automation needs. If a conversion from Maven is something that Gradle intends to support, then I'd say that's right in the core `gradle` executable's wheelhouse.
Supporting such features suggests that Gradle's CLI needs to expand a bit. Perhaps 'everything is a task', but as-is, that wouldn't be too friendly to the user. Hans and Adam have already been discussing this, but Gradle needs to somehow accomodate things like per-task options, a structure that feels like Git's subcommands, etc.