It's not just corporations adopting blogging, but some are toying with podcasting. Large corporations generally have some audio or video production crews already, so it's a natural extension for the output those crews make to be distributed on the Internet using the podcasting mechanism.
In Corporations podcast their marketing nets we see an overview of what's going on with corporate podcasting. It really runs the gamut.
The article also discusses how some attempts, e.g. GM's, have come across as too much like a cheesy marketing piece rather than an authentic podcast. Okay, fine, whatever. Podcasting is just a medium for distributing content, and that medium can be used for whatever some webmaster wants to use it for.
I suppose the point is that for someone to listen to a podcast they have to find it useful. Would you want to listen to many minutes of marketing schlop? Well, okay, I think the answer might be in all those infomercial programs on television. For some reason people watch them even though they're obviously biased.