My thoughts on this move were summed up (and partly formed lol) pretty nicely by Dwyer over at Ball Don't Lie if you care to check out his post.
Basically, if they throw the max (or near) at Smith, theyre kind of heading in opposite directions. That's a very young team with an old point guard who is a massively tradeable asset right now as his deal expires next summer. Why hang on to Miller if they're reloading with young guys? It's still difficult to see them getting out of the first round with their roster plus Smith. Sure they'll be real fun to watch, and Miller is a good point guard, but once he leaves next off season (assuming he won't take a huge pay cut) they're left with Sammy, Iggy and Smith eating up a huge portion of their cap room and they've got guys like Thaddeus Young and Lou Williams who are going to need re-signing.
It seems like they're handcuffing themselves over the next two offseasons (which are, as we all know, huge FA classes) to get a shot at Josh Smith. They could have moved Miller to clear even more space for the Smith signing, chalked this season up as a building year in which they would still battle for a playoff spot, and then have further cap room to work with over the next two summers. They won't want to trade Miller at the deadline this season unless the team is out of it, and all indication is that with Smith they'd be a middle of the pack team.
That doesn't change the fact that I will still watch the hell out of this team if Josh Smith is a part of it though.