I'm not entirely sure that there is as huge of a barrier to adoption from the corporate side. Office Communicator and Windows Messenger before that offered private network IM capability. Additionally, the Millenials and like-minded young adults are starting to filter into the workplace as interns and entry-level employees... Watching job websites, I often see articles about attracting the younger workforce. Microblogging is one way.
The true barrier is *not* the old school management thought (although it is a realistic barrier). The true barrier is old school employee thought. Microblogging escapes the mindset of even much of the 30-something crowd.
Another possible barrier to Yammer could be seen in a private network microblogging installation. Ultimately, this is Yammer's barrier to growth. However, much like AOL IM vs Office Communicator, grassroots adoption of microblogging will occur long before critical mass is achieved to provide a business justification for paying for or even just installing such software on a server and client machines.
The company e-mail validation on Yammer counteracts such software installs as well; however, a departing emploee would still have his or her access to internal company updates after departure, as long as the employee's e-mail has been validated. In Twitter, you can make updates private or block people from following you, but having to do this in Yammer would probably defeat the purpose it was created for.
Yammer, much like AIM, has the potential to put company information outside of the company’s hands. Of course, every user does need to sign up and verify through a corporate e-mail address. Moreover, those who have left the company can be requested for removal (one would have to wonder the validation steps here.) Nevertheless, there is no guarantee or auditing of the controls that would keep a company’s data private.