They gave Spike exclusivity (kinda), but I think if ESPN gave Dana a deal he liked, he would grant them exclusivity and jump off Spike. It's simply more eyeballs, and ESPN is the final step in acheiving "legit sports status".
Would ESPN play hardball with production the way HBO did? I don't know the answer to that, but that's an issue Dana won't budge on (and he shouldn't because UFC does a great job producing their own product - it's first rate all the way).
UFC hype machine + ESPN hype machine = unstoppable. The question, is could Dana handle ESPN covering other orgs, and if not, would ESPN risk journalistic intergrity (ok, ok, I know, but play along) for the sake of ratings?