Sorry for the double post, I wanted to finish my thought and hit the wrong button
Microsoft would be stark raving mad if they didn't
a) Improve IE Mobile to make it vastly superior than what it is now (maybe that's what's holding up WinMo 6.1x? We were supposed to have that already)
or
b) Allow MIDs makers to include other browsers on the device as they already do with phone makers.
Microsoft is probably looking at the big picture of having Windows Mobile on as many devices as possible as opposed to the individual components.
I'm inclined to think the delay with the release of the next version of WinMo has something to do with making more improvements now that the iPhone is having serious traction and they have had a chance to see what others are doing.