I've been carrying my mini in my pocket every day since I got it in May and it's fine (apart from the dent where I dropped it once onto a tiled floor).
I'd say two things: if you keep it in a pocket, just make sure there's nothing else in that pocket (keys, coins, etc.) and secondly, when you handle it (to change songs, etc.) make sure you take a little care.
(The nothing-else-in-pocket thing is good advice because not only do you eliminate the risk of coins scratching the iPod but also the risk of dropping your mini while you fumble for change... It's the one-mousebutton theory: do one thing at a time)