I know you're able to do this for email opens and clicks. I am also aware of the user agent field within the activity log of an individual record, but is there a way to pull this as a full report (list of leads) per day, week, month or quarter?
You can approximate a smart list of folks by constraining the filter "Visited Webpage" by Browser. You can then try values such as "mobile," which will get you any visitors who were using something like Mobile Safari.
You can also reconcile that list with whatever your Google Analytics account says about web traffic.