Ryan Farley has written a nice article about making fields readonly. Ryan has written several good articles from a developers point of view. I expect there will be more good work coming from his hands. Welcome in the world of CRM bloggers Ryan!
Most of the time I'm working on extending the possibilities of CRM for Cherry-T's clients. You'll find me at the Microsoft CRM Discussion/news groups. If I'm not there, you'd better start searching in bars in the neighborhood of Amsterdam :)
O yeah, the disclaimer:
1) Please note that these are my personal blog posts and that this does not necessarily reflect the way my employer works.
2) Most of the postings are about unsupported modifications. If you do apply these changes to your implementation, then I, my employer and Microsoft cannot be hold responsible for any issues that it might cause.