One approach to credit is simply to Tell People What You Did. This is applicable to wiki, and to life in general. When it matters, citation can be usually sourced and demonstrated.
This is the way most things work in life. When presenting the work of a group in your organization, the ideas aren't tagged according to who originated them, and the report you pass out doesn't specify who did the spreadsheet and who came up with the tagline.
Instead, the work is attributed to the group, and you are (hopefully) empowered to Tell People What You Did (specifically) at times when getting the credit would help you -- talking to your boss, writing up yoour yearly review, applying for a job, getting drinks with colleagues.
There are issues with this approach, however. Many speakers and writers have had the problem that an idea they have been presenting for years suddenly becomes associated with someone else. How to present that idea now without being accused of plagiarism, and without getting into a very defensive "yes this is really my idea" discussion? It's not easy.
Commentary
Personal note, when I was writing for Blue Hampshire, we were chosen to be one of eight sites in the U.S. featured in a new Newsweek project called Ruckus. The writing on our front page (which was generally produced by me and Dean Barker) was syndicated to the U.S. Newsweek audience directly, a very big deal, first time in the history of political blogging for something like this to happen.
For various reasons, the Newsweek feed architecture only supported single authors, and so my posts were filed under Dean's name, and his face associated to my insights and prose.
It bothered me a bit, but eventually I realized I was pretty blessed. The site Dean and I had created had resulted in us making history. I knew it, and I could tell people to whom it mattered the truth.
Had we cared about the credit, none of that could have happened. Maybe part of not caring is to realize these things are not zero-sum. -- Mike Caulfield