Jeremy Wright has posted a list of the Fortune 500 companies with blogs. He counts any type of blog -- internal, external, product, etc -- as sufficient for a company to be included in the list. Jeremy, thanks for keeping us all up to date. (Assist to Steve Rubel at Micropersuasion)
There is also TheNewPR/wiki that's keeping track of corporate blogs (listed by type of corporate blog as well) and other PR related developments. The list is not as comprehensive as Jeremy's but there many other good resources included in the wiki.
