Because I'm totally clueless with how corporate America functions,
Remember that corporations make decisions through people, human beings authorized to speak on the corporation's behalf. Accordingly, corporations are as unique as the people who make decisions on their behalf. There is no single, unified "corporate America" with predictable behavior.
once I file a patent app, and I go to say, MSFT asking them if they r interested in my technology, what do I have to stop them from practicing my invention? My understanding is that they don't need to worry about infringement until my patent issues.
You have two and only two options to control the behavior of MSFT (or anyone else): issued US patent or enforceable NDA.
Also, who at MSFT do I approach and how do I go about doing that? If I want to use a middle man (IP broker firm), who are they and how do I contact them? Can I offer them an equity share for covering the patent costs? Are they likely to sign an NDA prior to me filing a patent app.
Others here have provide links to articles listing some of the patent hedge fund players. Note that they won't go to MSFT on your behalf. If they do anything at all with your patent, they'll buy it from you and assert it on their own behalf.
The idea of approaching MSFT is not that you have the power to stop them from using your technology now but rather than they might be very interested in the ability to stop Google from using your technology in the future. So, they might want to buy your technology from you (including any pending applications) in hopes of asserting whatever rights you could have acquired. In order to have those rights some day, they might want to purchase those rights from you now rather than later, despite the application being merely an application at this point.
Of course, Google might be willing to buy your technology just to keep MSFT from stopping them from using it. There, you have a bidding war. Lucky you.
Naturally, this assumes that your technology is "all that" and both Google and MSFT believe that.
As to whom to contact within MSFT, I would look for someone related to Bing and on the marketing side. But, you have to have your pitch that your technology will provide so much better results than Google's that Bing will grow market share, and the pitch has to be very persuasive.
And, lastly, remember that all this tech transfer stuff is beyond what I do. This is all speculation and conjecture from an interested observer of these sorts of things.
Regards.