I'm looking for an experienced mobile UI designer to lead the vision and design for our iphone and android applications. What characteristics, experience or requirements would a mid-to-senior level mobile app UI designer have to have?
You'll have to decide if you want that person to really be a UI designer or if you want them to develop the code as well.
In my experience, it's best to have someone focused on the customer side of UI/UX who works closely with a true UI developer to make the magic happen. Two people are definitely stronger than one in this role.
If it's a visual/experience role, then you want to look for someone with a head for information design and experience with not only great designs but site optimization. Someone who has experience with the iterative process of unearthing expected ways to improve your metrics.
It depends on all sorts of things. Personality is often the big one, followed by the quality of their resume/portfolio. Beyond that, how big is your team? Will this be a one person UI/UX person or is there a bigger group? Do you need the UI person to design? Develop? Both? User test? Software test? etc.
It's really all going to depend on the specifics of your product, your team, and the project.