Well HDMI 1.3 may well bring improvements. But there is no receiving device available right now that has that capability - so we just can't tell.
If Pixel Magic wanted to put HDMI 1.3 in there today - they probably couldn't due to lack of supply + it would take quite a bit of time to come to market and iron out all the bugs, etc.
Last I heard even the Lumagen which is due in a few months time wasn't going to make it with HDMI 1.3 at launch although they have stated there will be an upgrade path.
Before it will make much of a difference, we will also need display devices with HDMI 1.3 too, and again - these have no firm dates + supporting HDMI1.3 doesn't necessarily mean it will have all the potential improvements.
At the end of the day - if you want to buy a VP today you have to choose between the HDP/HDQ, VP50, Crystalio II and Vantage HD. It's a great choice to have!
Madshi's comments are always very well observed. 8bit is the current input limit of all the current display devices, so it's not really something that I would worry about for now. Even Madshi has decided not to wait for all these new features indefinitely and took up DVDO on their upgrade offer for the VP50 I think (Madshi please forgive me and correct me if I'm wrong).
There will always be something just around the corner. If you can wait, then great - personally I'm sticking with my 768p crt projector until digitals beat it in EVERY area - I feel comfortable in my decision - and ultimately you have be comfortable with yours.
Regarding the 3800 model - tough call - I'm not sure that I'll be going with HD-SDI unless it becomes more available and to be completely honest, the HD Mediabox looks 90% as good via HDMI and currently supports more formats - but it's a little less automated (resolution detection). However I will ditch it as soon as the internal one adds .avi support.