I think giving your word means a lot between GMs and agents. I know when Carlos Boozer backed out of his verbal agreement, his agent felt a ton of pressure.
Or, look at the Marquis Daniels situation. Both the agent and the team seem happy with the arrangement, and both sides are acting like they are bound by it.
On the other hand, you've got the Hedo Turkuglu deal. I think he and his agent probably made a lot of enemies with their decision to back out of a deal. If there's any gray area, it's that Hedo's signing came during the July moratorium, when teams can't officially sign players.