The issue I've run into with corporate lawyers (that tends to get branded as "co-employment") is whether there is a perception of employment. Their concern is that you ask a non-employee to do something specific with a specific, tangible reward for that something, then you are setting up a situation where they appear, by all accounts to be an employee. Think about that for a sec: what's an employee? It's someone you task with specific tasks, then pay them for doing those tasks.
So the corporate lawyers tend to fear a single volunteer lawsuit, or even worse, a class action lawsuit from a group of volunteers that make the claim that they were asked to function as employees and therefore are entitled to backpay/benefits/protections.
There is existing case law, although I'm not sure how much it really applies. The biggies are AOL and About.com.
The way you work around this is to ensure that you're not saying, directly or indirectly: "If you do X, I'll give you Y". Think about it as an award vs. reward.
Reward: "If you volunteer, I'll give you a laptop"
Award: "Because you did such amazing things last year, you've won the Fan of the Year award which comes which comes with a prize package"
Am I making sense??