Just hooked up my new HD-A1 last night to my Denon 4306.... it's amazing! There are no compatibility problems whatsoever... I'm able to get the video and DD+ over the HDMI (which the 4306 recognizes as multi-channel PCM).
Like many of us on this board, I spent way too much time agonizing over "specs" before buying these two products..... whether I should wait for HDMI 1.3, whether I should wait for Dolby TrueHD support (in the receiver and the player), whether Blu Ray would be better, and on and on. For anyone with these concerns, my $0.02 is that you should stop worrying about the specs and just pull the trigger. The picture on the HD-A1 is incredible, and I'm using a plasma with less than a full 1080i resolution, so I can't even imagine it on a better monitor. The sound from DD+ is so good that I'm really not nearly as concerned with whether this setup will handle TrueHD eventually.... it's hard to imagine that my ears could pick up on any improvement over the DD+.
It's really changed my perspective on the format war too..... I'm now really irritated that the only thing that's keeping me from fully enjoying this player is the ridiculous format war. Hopefully, it will be resolved one way or another soon. In the meantime, the $500 I spent on the HD-A1 was well worth it, even if it's ultimately not the prevailing format.