Estimating license adoption is an inexact science. 2003 through 2005 we relied primarily on Yahoo! link: queries (Google's link: operator obtains very incomplete results). In late 2005 Google added CC queries via their advanced search page and also via their API (Yahoo! also allows this, but in this case Google's results seem more comprehensive).
We can also know the number of works licensed at various Content Curators. The largest of these based on recent (early April 2006) for various formats may be:
- Images -- Flickr -- 10.8m
- Sound -- Soundclick -- 220k
- Video -- Revver -- 19k
A catalog of past estimates, with pretty graphs forthcoming.
- We'll publish a new estimate in June.
- Based on Google queries, this number was only posted to http://creativecommons.org/support/ which has since changed but is reiterated in two published Lessig interviews in January.
2005-08-09 53 million pages licensed 
- Again based on Yahoo! queries, this number turned out to be overstated as Yahoo! tuned their results estimation after growing their index.
2005-06-13 CC search query breakdown
- Breakdown of search requests and desired license properties -- people searching for video want the least freedom.
2005-03-07 CC search index breakdown
- Breakdown of (small) CC-nutch index -- audio publishers are most permissive, video publishers least.
2005-02-25 License Distribution 
- Based on Yahoo! queries there are now 10m licensed documents. Pie chart of what those licenses are.
2005-02-18 How many pages link to a CC license? 
- Based on Yahoo! queries, 5m. At the end of 2003 it was 1m.