You need a lawyer when...

Dave Winer regrets not having had a lawyer review a deal.

Get it in writing, and have a lawyer review it. Ask Jason Calacanis, who wisely says if a deal is worth doing, it's worth having a lawyer write it up. If it's not worth that, then don't bother.

Dave writes from the perspective of someone who feels he was taken for a ride, but it's also good to have a lawyer involved even if the people on the other side of the deal have the utmost integrity. Honest misunderstandings happen, and a good lawyer can help avoid them before they happen. No one can avert all possible misunderstandings, but one can reduce the risk and, in the process, increase the likelihood that the agreement between the parties has the intended legal effect.