I wouldn't touch iCuke, it's a dead project as far as I can tell. Calabash is basically a clone of Frank, to avoid some GPL-ness. It may merge back into the Frank project, and doesn't have the community support Frank has.
I quite like KIF, but I'm not sure how much Square are supporting it. I spent a day with the developers of KIF 2 weeks ago, and they seem to be waiting to see what Apple releases in June around automation before committing more resources to making it better. With so many ex-Apple people at Square, I got the impression they knew something I didn't! Square have some nifty extra tools they use internally that haven't been open sources yet, but would make KIF a whole lot better.
KIF does the job, and is suitable if your team is comfortable with using Objective C for high level tests, and your app doesn't have complex interactions with a backend system. Frank has by far the most active development and community around it, but does add complexity to your test environment. Frank is better when your app does have complex interaction to the outside world, and when your team is comfortable using Cucumber and Ruby already. UIAutomation sucks, but has good device support, and is always going to be up to date with the latest and greatest. Zucchini makes UIAutomation nicer if you don't mind needing to know CoffeeScript, Javascript and Ruby to write your tests.
Luke - the idea of having a common set of test scenarios, with different implementations for iPhone and iPad is something Frank/Cucumber would give you almost for free, just by having separate step definitions. I know projects doing this with common high level tests across iOS, Android and mobile web, with the step definitions implemented differently for each platform (using Frank, Native Driver and Web Driver respectively).
- Stew