Connie Crosby assembled a bunch of information on wikis in law firms, including a slideshow and a summary of a roundtable discussion: Do Wikis Belong in Law Firms? You can follow the link over to Connie's blog to get the details.
One item from the discussion caught my eye. The person was surprised that there was "any wiki use in law firms since they see the culture to be competitive, not one conducive to sharing."
I think you need to make the distinction between lawyers and the law firm culture. Sure lawyers are competitive. It is a harsh job, demanding lots of time and knowledge. In law school everyone realizes that those with the highest grades will have their pick of jobs and those with lesser grades will find it harder to get jobs. Lawyers are competitive.
But law firms are about lawyers organizing to work together and share resources. Partners need each other for their expertise across subject matters. Partners need associates to help get the work done. Associates need the partners to bring in work. Everyone needs their secretaries to help get the work done. The lawyers need the library to staff to help research. More experienced lawyers need to share their knowledge with the junior lawyers so that the junior lawyers can get the work done properly. And so on. The better the sharing of knowledge about the work and sharing of skills, the better the firm will operate.
Sharing your knowledge with others in the firm does not diminish your value to the law firm. It increases your value. Sharing your knowledge just exposes your expertise. You will always know more than you can write down. Whatever you manage to write down and make findable will act as signpost for people to find you and your expertise.
You can hold yourself out there as an expert. But unless people hear what you have to say (or read what you have written), they are not going to buy into your expertise.