Check out this video of a Nokia 5700 smartphone running BlackBerry OS 4.6. Who knows how they got this to run, but they did. If you just look at a still shot it could be fake, but this video seems to indicate that it is in fact, real.
The video shows that some guy ported the BlackBerry Pearl 8220's build of OS 4.6 to this Nokia 5700 and got it to work. Why would anyone want to do this? Your guess is as good as mine.