Was rooting around for some information and stumbled across this page which may be of interest:
http://googlereader.blogspot.com/2006/08/namespaced-extensions-in-feeds.html
Namespaced Extensions in Feeds
Thursday, August 03, 2006
posted by Mihai Parparita
“I wrote a small MapReduce program to go over our BigTable and get the top 50 namespaces based on the number of feeds that use them.”
| % of Feeds | Namespace | URI |
| 29.36% | Dublin Core | http://purl.org/dc/elements/1.1/ |
| 0.21% | PRISM | http://prismstandard.org/namespaces/1.2/basic/ |
Seems quite an impressive percentage for PRISM.