Just thought I'd add something..
Since i've been out of sparring for a couple of weeks now due to the neck injury mentioned in the OP I've been trying out a bit of 'ashtanga' yoga. It contains alot of bridging poses off of the shoulders and head and ends with a sequence involving headstands.
As far as I can tell its helped quite a bit with my DIY rehab and it contains all of the other obvious benefits of yoga (flexibility etc).
I would suggest that this is probably a great alternative to using actual weights on the neck and/or a good way of building up some preliminary muscle and tendon strength before the use of weights.
If you can't find an instructor you can get a practice guide on amazon: Amazon.com: [Ashtanga Yoga: The Practice Manual][Swenson, David][Hardcover]: David Swenson:
as well as DVDs: http://www.amazon.com/s/ref=nb_sb_ss..._0_0_1.202_158
What we do in our boxing gym is a worm up thing, we stick our fists under the chin and stretch the neck backwards, and apply some resistance while crunching down the chin towards the chest. For the back of the neck we simulate a clinch grip just applying it yourself, and work the full range with resistance that you feel good with (The Same thing for the sides). When I did judo we used those bridges and that was more than enough. Lately I started using a big rubber ball (the common one u see in gyms). Press it to the wall with your head in a fighting stance and you can move slightly all the time pressing into the ball. The angle of the stress simulates perfectly a blow coming to your head while you advance in a low crouching stance. The relatively low intensity in this exercise enables you to utilize full range without fear of injury. If you just starting to condition your neck I would urge you to start with one set for each exercise and see the next two days how you feel. If you didn’t suffer any soreness of the neck muscles than progress to 2,3 sets. If you feel soreness afterwards, (the good muscle kind) than start with twice a week.