As a long-time Wizards fan, I have to try to temper your enthusiasm a bit... Butler is an underrated defensive performer (not great, but he works at it), but Haywood is vastly overrated defensively...
He has improved somewhat as a rebounding force this year, but he offers next to nothing offensively... and while he averages 2 blocked shots a game, he's not really a force as a shot blocker... the blocks he gets basically come to him... he's not particularly active defensively, he just kind of stands around...
Don't misunderstand me, I think the Mavs have improved themselves with this deal, and I hope that Brendan and Caron get to make an extended playoff appearance... but I've been hearing all day about how Haywood is "one of the better defensive centers in the NBA" (more ESPN hype), and all I can say is I watch the Wizards every chance I get, and I haven't seen it...
And for the Wiz, the rebuilding begins... don't be surprised to see Jamison traded in the next day or two, my guess would be to the Cavs for Ilgauskas (another expiring contract), and perhaps JJ Hickson, who they reportedly like a lot... and I have read that the team is looking to break up the remaining nucleus of Andray Blatche, Javale McGee, Nick Young and Dominic McGuire too... they reportedly offered Blatche to Charlotte for DJ Augustin, and were rejected... but surprisingly enough, despite the conventional wisdom, I'm reading that they may not void Gilbert Arenas' contract... I can't believe it, but that's what they're saying...
This likely means the Wizards will continue to struggle this year, which means a lottery pick... if they can FINALLY get lucky in the lottery, that might mean a John Wall at point guard, or an Evan Turner at big guard/small forward... and they'll be well positioned cap-wise to pursue a name free agent in the offseason-- Dwayne Wade (unlikely, but I can dream), Chris Bosh, Amare Stoudemire, Joe Johnson, perhaps even Dirk Nowitzki could be on their wish list... I like Nowitzki best of all after Wade from a pure talent standpoint, but the others are considerably younger...
If they stick with Arenas, then Wade or Johnson would make less sense...