Tricky question(s) regarding NDA ;-)
But I'll try to tell something without wanting this to end into a massive thread of questions, or speculations flying around. Although some people will not resist the chance I guess...
So the following is as-is, no follow-up answers on this post, only pretty pictures... ;-P
Like all other 3rd party renderers for SI out there, most of the SI native shaders are supported and Arnold comes with it's own set as well.
Some of the the native SI shaders are not really used due to the way Arnold works.
There's 3rd party shaders available, like the BA shaderset, Amaan's Ocean stuff, the sIBL toolset from Thomas Mansencal, Royal Render support etc. Works all wonderfully and quickly.
Regarding the mia shader, yes, there's a more or less similar shader available that is tweaked/improved all the time based on customers requests & comments.
Some people just create some of their own specific shaders, like Digic for instance. But that will happen with MR or Renderman as well.
Keep in mind some of the MR shading/lighting workflow isn't necessary to get to similar results, or some workflow just needs a little 'rethinking' as like working in VRay or 3DLight for instance.
But most things are pretty straight forward, and the lesser amount of 'render knobs' within Arnold are a pleasant departure from MR.
Most of the ICE stuff is supported, some is worked on to get it into Arnold asap. Volume rendering, and I think you mean smoke like stuff, interesting things on the horizon but NDA... sorry.
Eric Mootz & Holger Schonberger are working on volume related stuff that, besides for MR, will be available for Arnold as well, so it all looks very promising on that end too. More choice the better.
There's a very active development on both the renderer, including work done at Sony, and the connector.
Sure, like all software there are things to improve & tinker with, but most of the Arnold users are happily up and running within a very short time.
Oh.. release date for 'the public'... No one knows
, so no use to speculate or ask on this one over and over again. The massive speculation thingy is looming here quickly.
Only thing I can say, if you're a company and really interested in testing Arnold & buying licenses
, follow the emailadres on their webpage. The chaps at SolidAngle are a great bunch and very helpful.
As a freelancer, if you want to render outside MR now, there's 3DLight and the beta for VRay atm. Just saying ;-)
Again, please no follow-up with extra questions, I know that sounds harsh, especially for freelancers. But I cannot tell you more than this, sorry. When more info arises and becomes publicly available (non-NDA), I'll post it here.
As a peace offering here's the videos of the presentation Marcos did on the Uebertage 2010: http://www.vimeo.com/15878348
And these chaps used Arnold straight after buying it, on this commercial: http://vimeo.com/25464907
See... back to the pretty pictures again ;-)
@ HB: URL tags again this time!