I had the chance to listen to the HP-P1 at CanJam. I was using my own iPhone 4 with up-to-date firmware and software (I had not upgraded to OS5 at that time - still haven't actually). I really liked what I was hearing from it, but for the pretty regular, very subtle clicking. I asked the Fostex guys who were manning the table. One spoke no English at all, while the other was conversational. They both listened and confirmed it was there. They asked about the firmware, checked the phone and installed software and apps, made some phone calls speaking in Japanese, and could not come up with anything beyond that they had experienced some examples exhibiting this behavior and it was usually because of dated firmware. This was not the case with mine and they were both stumped and said that they'd report the incident. It kind of spoiled my experience and my interest in getting one, which is a shame because, other than that clicking, I liked the output into my Westone 3's better than straight out of the phone (go figure) - it was very clearly superior to my ears.
Has anyone had similar experience with the Fostex and figured out how to clear it up, or ended up returning it because of such a problem? My guess would be that it has something to do with the DAC locking onto the signal. It was definitely a digital artifact of some sort...very subtle so you could only really hear it on softer passages, but it was clearly and consistently there.