I suspect that RIM went to bed with Kobo (another Canadian company) and granted them an exclusive for some amount of time - at least 6 months. Amazon would love to have a Kindle app on the PB - their model has been to write apps for as many devices as makes sense. I'd bet good money that RIM is blocking them from App World in favor of Kobo. I don't expect we will see the Android Kindle app ported over once the Android emulator goes live "this summer."
The problem with that is that there is Zero chance that most Kindle faithful will join up with Kobo and start all over again. I know I won't.
I bought my PB expecting to see a Kindle app at launch, as was advertised months ago. Mine is probably going back to Staples later this week.