It's difficult for me to care about this trade too much. Prior to the trade, the Mavs would have been the favorite against the Spurs in the playoffs. The Mavs were built perfectly to exploit the Spurs' weaknesses -- specifically the lack of size and mobility next to Duncan.
Now? It's pretty much the same. Instead of having just Dampier to throw against Duncan, they have Dampier and Haywood ... which obviously isn't good news for the Spurs. Against most teams, Butler is an upgrade over Howard for the Mavs. However, against the Spurs, I'd say it's about a wash. The Spurs could never guard Howard because they didn't have the right type of athlete to put against him. On top of that, Howard played with the utmost of confidence against the Spurs. As it stands, Butler is the better player but I doubt he'd be too much of an improvement over how Howard routinely destroys San Antonio. Plus, what does this trade do to the minutes of Spur Killer Jason Terry? That's not too clear at this point.
I still give the edge to the Mavs in a series against the Spurs. I don't think their odds improved that much from pre-trade though. Thus, I won't lose any sleep over this trade. The Spurs have much bigger problems than a team that already would have beaten them moving around some talent.
In the big picture, however, this was a very good trade for the Mavs. Before the trade, they had no shot at a championship. After the trade, they are still huge underdogs but there is a chance. Caron Butler has never played with a legit ball distributing point guard before. Kidd could help him blossom and reach his full potential. Going from playing with point guards like Arenas and Chucky Atkins to Kidd can do nothing but help.
Haywood will also help, especially against the bigger and tougher frontlines. Motivation and effort has been a problem for Haywood at some points in his career but it being a contract year should make that a non-issue. Not to mention how he'll be going from the worst passing team in the NBA to one of the best.
The best part of this trade for the Mavs is that it doesn't hurt them long term. They have this year and next year to give it a go and then return to having flexibility. Usually in this type of salary dump, a team has to mortgage their future. In this deal, Cuban just basically bought a chance at a championship without busting his payroll structure.
The Lakers are still the huge favorite in the West. I'd say the Nuggets remain second. After the trade, the Mavs are a solid third. The Jazz might have like a 0.1% chance ... but I give no chance to the rest of the Western Conference as it stands.