EINs are employer identification numbers. They are essentially the corporation equivalent of the SSN, but they are not considered to be sensitive information.
Wikimedia's EIN is 20-0049703.
It's shown on the letter at http://upload.wikimedia.org/wikipedia/foundation/a/aa/501%28c%29%283%29_Lett... and on sunbiz.org and has been mentioned on meta - http://meta.wikimedia.org/wiki/Talk:Fundraising_meeting,_September_2005#5:_C...
If you can think of better places to put it, please do.
Angela.