yes, i have. my friend stopped using his account and they killed it. he tried to call them to open it back up, but they wouldnt do it. this was early last year.
I would think they would eventually...after all, advertisers are paying for traffic when you get down to it, if they arn't seeing traffic, they will quit advertising, YPN will lose them, and then they will look at you for a low conversion rate...
eh, if you arent using your account you arent charging the advertisers money or a lack of conversions. i think they just shut them down to make room for another publisher that will provide them with activity to do their beta test. that is jmo
I've also heard of them closing accounts for non-activity. I have also heard of Adsense doing the same.
I would make sense to eliminate the accounts with no activities, since it probably takes some time to manage/monitor those dead accounts. However, I did have my ypn account stay inactive for about 6 months before deciding to give it a try.