The restrictions I remember with the ROKR was that the iTunes component wouldn't deal with more then 100 songs, to try and prevent the ROKR from eating into the iPod sales. The majority of iPods sold are the lower end flash based ones, so a phone that could be expanded into their range would be a threat.

Now that the phone is 100% apple controlled, they don't have a problem with this. I'm betting the Motorola alliance was one to allow Apple to learn the phone business a bit before jumping in with both feet. Similar to how Microsoft worked with Sega on the Dreamcast before going ahead with the XBox.