Earlier in 2007 Blizzard released the Armory, which made the data of all their servers available to the public. I have used the Armory a few times to provide information on my character(s) to friends. However I had not looked into data mining or other statistical usage from the database.
However, in general surfing around I stumbled upon some resources and folks who are already neck deep in data being mined from the Blizzard armory for World of Warcraft. The following are a few of those links and sites and I am sure there are plenty that I have not found yet.
The Build Mine
Llew Mason's Musings on Data Mining
WoW Insider's write up of Zyph's data mining
Zyph's Forum Post regarding his data mining
This next link is not necessarily related to mining the armory, but is a nice statistical and charting effort of changes to the character classes over time as recorded by a World of Warcraft player.
Charting WoW Balance Changes
The above are some quick links to some existing efforts and analysis, there exists great opportunities for tools to be built of mining data and analyzing based upon specific criteria. One such items are the crop of guild progression tracking websites. One i have used frequently is
Beyond this there is the possibility of tool sets to aid guild recruitment for PvE and PvP and countless other utilities and tools.