Looks like I'm the only one who disagrees, but I say keep Burnett. Great pitching is a lot tougher to come by than a great outfielder, and Burnett has been great so far this year: 2.54 era, 1.13 WHIP, 5 wins, 43 K's in 49 innings. Burnett is young, 25 years old, so despite his large pitch counts, he should be better equipped than, say, Greg Maddux, to take that pitch count.
The major factor here is that if you trade away Burnett, that leaves your pitching staff with Clemens, Colon, Wood, Fogg, Ishii, and Moyer. While not bad, quite good actually, that staff's has a chance to be a little too erratic for my liking, and could self descruct. While this may be possible for Burnett too, at least that gives you one more option or possible superstar.
Despite his incredible season last year, it's likely Moyer will be an ERA liability with some wins, Ishii a WHIP and ERA liability with lots of K's, Fogg is awesome now but he's still proving himself, Wood's been erratic but pitched an incredible game tonight, Colon has been awesome and should remain so, and Clemens has been merely good. Incredible pitching staff for a real team, a good pitching staff for fantasy, but the goal is to be great.
So to sum up, I say make this trade only if you're DESPERATE for some batting help, or try packaging Ishii with a reliever to give the owner that final push he needs. Of all your pitchers, I believe Ishii is currently the most overvalued.