In what way are you thinking? As in, identifying users who are using mobile devices? There are two default dimensions for this... Mobile Device (this actually breaks out all the devices themselves - iphone don't really get broken out due to Apple hiding this info) and Mobile Device Type which, despite the naming, is used to identify all the different devices types (other being desktop)
If you are trying to get more granular with the data, Y=you should be able to breakdown or segment your UV metrics by these dimensions... but you can also add custom eVars or props to your tagging implementation to capture additional data that would support your needs.
I would love to help you more, but I need to know more specifics about what kind of evaluation you are trying to achieve.