About half of all my drives are the Ultra 320 (I have over 160 drives in my servers) I have not noticed a higher rate of failure in these drives compared to the Ultra 2 and 3 drives. Personally for me, the late model Ultra 3 drives (especially the 36.4GB 10k RPM drives) seemed to fail a lot. I have replaced six out of fourteen drives from one array. However I do remember seeing an advisory come from HP regarding Ultra 320 drives late last year. You might want to search the HP website to see if you can find it.
Regards,
Ben