Adding social bookmarking tags to Lotus Quickplace
One reason for not using tags on an intranet is that only the author are likely to tag their article, as opposite to the social tagging systems on the public internet where actually everyone is supposed to tag everything (that’s of there interest). If we can get an habit to have intranet users to tag everything they read with accurate tags, then the article would more likely be tagged correctly. IBMer Satwik Seshasai has an article (Adding social bookmarking tags to Lotus QuickPlace) posted back in May on developerWorks about this, rather cool indeed!

Stumble It!
Multi-level categorization of documents is an easier way to accomplish almost the same task. Categorization shows where in the hierarchical structure the document belongs, so people can rather easily locate the category they are interested in. This can be seen as if a document has only 1 tag, but a VERY relative tag.
The biggest problem with tagging of Intranet is probably that people will tag documents which are interesting/fun to read, not which contain boring valuable information. This will produce many irrelevant search results and increase time to find the information.
It would be interesting to see if there will come some Domino tagging applications in the future. In some types of databases tagging can be rather usefull.
[Very long answer moved out and will be put in a new post within som time (hours)]