This is directly from NY-NJ Trail conference map concerning camping in the Catskills:
"Camping is permitted on state land below 3,500 feet in elevation. Except at lean-tos and campsite specifically designated by the DEC, you must camp at least 150 feet from roads, trails, and water sources."
From my fairly limited time in the Catskills, camping does seem to be an issue. Most of the peaks (like the three I passed on the Devil's Path) are over 3,500 feet so you can't camp there. I passed several other areas below 3,500 feet that would have made nice camps (and look to have previously been used as camps) but those were all posted with "no camping" signs. And getting 150 feet off trail in the remaining areas would require serious bushwacking and even then, finding a flat, non-rocky spot would be tough.
Maybe I just haven't been in the right places, but even though the regs make it sound like you can just pick a spot, it's not that easy. They seem to force you to camp where they want you to without actually spelling it out. That being said, I've been on a few trips up there over the last year and have rarely ran into anyone else on the trails or camping so I wouldn't let any of this discourage you from making the trip - it's still well worth it.