I read about a report How are corporate weblogs affecting public relations practice? on the weblog of Neville Hobson. The conclusion of the report is great I think:

"The findings of this research strongly suggest that corporate
blogging can make a substantial positive contribution to the practice
of external corporate communications. The companies that are blogging
successfully are reaping the rewards of better relationships with their
stakeholders and enhanced corporate reputation. Controlling the risks
of such unfiltered dialogue between employees and the outside world
using a combination of trust, policies and guidelines is an enlightened
act of management. This is a significant change from the
command-and-control approach to corporate communications that was
widespread in the past. However, these companies will not be closing
down all other methods of corporate communication to concentrate solely
on blogging, since blogs are additive to existing media, not
replacements for it."