At least according to this Rocky Mountain News article. About 70% of NBA players are against moving the minimum age to 20. 10% are undecided, and 20% are in favor. I don't think a straight limit is the way to go, though. Having Lebron in college would be pretty farcical. My solution? Glad you asked.
I think the correct idea is to let anyone 18+ enter the draft but to only allow them to be drafted by a certain point. If you're 18 you have to go in the top 5. If you're 19 you have to go in the lottery. If you're 20 you have to be a first rounder. Past that it's open season. This would require the NCAA to alter its eligibility rules a little bit, but they already allow high schoolers who are passsed over in the draft to retain eligibility, so it wouldn't be a huge philosophical leap.
Another thing I think would help would be revamping the draft status to something similar to what hockey has. Almost every forward on Michigan's hockey team has been drafted by the NHL but since they haven't employed agents or signed contracts, they retain their eligiblity. If you're drafted by the NBA you can't play college basketball any more, even if you didn't retain an agent. The NCAA used to justify this by claiming that hockey players were automatically entered in the draft when they hit a certain age and weren't opting in. This caused most NCAA hockey players to be drafted a year later than their CHL and European counterparts. Recently, however the NCAA allowed college players to opt-in early and retain their eligibility, so the NCAA's rationale for not applying a similar scheme to basketball has disappeared.
Though the NBA players seem opposed to it, since it doesn't really affect anyone already in the league, if the owners dangle a carrot, any carrot, in front of the players in exchange for some age concession, look for them to abandon their principles faster than Bill Self at a Luther Head court hearing.
None of this helps the Pistons with Darko, but they're NBA Champions, so shut up, Bill Simmons.