This may also work on iPhones, but I have a Samsung Galaxy S Fascinate, and when using the Pandora application it doesn't stop after only 1 hour like on the computer, and I can leave it on all day every day. When I look at the hour counter from the browser on my computer it doesn't change.
I don't know how long this will last, but for those of you that listen to Pandora while you work, and have an Android phone this is a good way around the time limit.