Hi Chris. The D1 is in my GF's truck. She'll be over in about an hour at which point I'll take a picture and repost.
Ok, now to your questions. The D1 was the 05 model, the D2 is the 06 model. The D3 is coming out in March 07. Yes, there are 2 slots, one for the map DVD, the other for audio/mp3 CD. You can get updates from Pioneer, or you can buy them cheap on ebay... (most of the ebay DVDs are burned copies, but they work) Or you can contact me for the 06 disc. ;-) Anyway, the difference between the D1 & D2 is pure software. Once you load the 06 software on the D1, it's 100% identical to the D2. The 06 software has map and POI updates over the 05.
My GF, Katie, has it in her Trac for about 3 weeks and loves it. In response to needing to pull over for POI searches, there is a work around. Do a web search for (avic bypass) and you'll see the easy workaround for this. The workaround fools the car into thinking it's pulled over at all times allowing her to use all the nav features while in motion.
The D1's are rock solid and can be found cheap on ebay. I just picked another one up, with the XM module. I think I'll install it in my mom's Toyota.
Installation will take some time, and you will need to dremel the lip edges of the dash trim. Specifically the inside of the radio opening... you'll see the lip once you remove your radio. You can't see it with the radio installed as it's behind the faceplate of the radio.
You'll also get other benefits with an aftermarket unit. Your overall stereo will sound better due to the added power and clarity. Her XM menus look great and display title/artist/category/channel info at all times on the large screen. I'll take some pics of this as well.
I also like the memo button on the D1. If you hear a song playing that you like, you hold down the memo key for a couple seconds to bookmark it. Next time it plays, no matter what source you're listening to (i.e. CD/radio, etc..), you'll see a popup telling you the memo'd song is playing and ask you if you want to switch to it.
I like this Pioneer unit much better than the Alpine IVA-D310 setup I have in my Honda Pilot. And it's much cheaper!!
Let me know if you have any other questions.
-Mark