Many, many moons ago I worked for a US multi-national and was a frequent traveller to the US on work-related trips. With a letter from my employer, I applied for and was granted a multiple entry, indefinite stay, B-1, B-2 non-immigrant visa. I'm not entirely sure these days with Homeland Security and the changed security requirements, but I believe you need something like this that allows you to discharge your duties to your employer while in the US but not to get paid from there. You do not need a work visa AFAIK.
I was advised (by someone who knows about this stuff) to keep the old passport with the visa stamped in it and bring it with me on US trips, which I have done, but have never needed it.