I recommend the Nokia BH-214, its got a 3.5" jack (use your own headphones) and media buttons and it works great with my n900... Pretty good battery life too from what I can tell as it has not died on me yet - gets used a couple of times a week! Has a built in clip on the back which works well too!

Highly recommended - its also pretty cheap, cannot remember where I got mine but it was around $40 back in december...