Much appreciated thanks ;) offically scared me off sit-ups now i think! Crunches are so dull..
http://well.blogs.nytimes.com/2009/06/17/core-myths/
This is an interesting article that addresses sit-ups specifically at the end and calls attention to the importance of form. I recently had surgery (ACDF) in my neck due to a herniated disc, Degenerative Disc Disease, and arthritis, and have worked out, strength training and cardio religiously for years. I have been researching the same thing recently and am finding that most articles will say I can do longer do either because of the strain on the spine. Livestrong.com is another great site with an excellent search engine. They have excellent articles on specific exercise information. Hope that helps!
This is a better explanation on the scientific end.