After my post, I got an email from the developer of an iPhone Twitter client. He was contacted by Microsoft a few months ago, with an offer to port his app to the Zune in exchange for "a bucket of money". He turned them down, but assumes, as I do, that Microsoft reached out to the developers of multiple popular iPhone apps.

My source is certain the offer was for the Zune, but because he turned it down early on, he doesn't know the details regarding the OS or SDK. If any other iPhone developers have gotten a similar pitch from Microsoft, I'd love to hear about it.