Since my startup is just getting off the ground, I can tell you more about interacting with companies via Twitter from the consumer's perspective.
I actually recently had a little spat with Verizon over a bill. I posted some sort of generic comment about my frustration on Twitter, and within an hour, the local Verizion account had @replied me to ask if there was anything that they could do. This led to a ton of phone calls, and general customer service stuff, and in the end, I think my problems are fixed. It surely makes me want to stick with them more than move to a different company, because at least they're paying some kind of attention, even if they're not ideal.
I think that Twitter for businesses should work the same way as Twitter for a person; it's a great tool to humanize a company. Just interact with people the same way that you would as an individual, talk to people, RT cool relevant things that you see, etc.
