If Corruption is delayed. I am willing to bet its so it can get online play. We saw a demo at E3 which probably wasn't the final build they had...and its been worked on for awhile since then. It should be infinitely better. We aren't talking about it, because we don't have any more information, and we all know what Metroid Prime is, and plays like.
Personally, I thought Metroid: Corruption would have been the perfect exclusive game to release multiplayer online madiness, since Brawl won't be out for some time.
The only other option is that Nintendo is gracifully backing Metroid Corruption back to give the 3rd party FPS more attention at launch. This would be a good strategy for 3rd party relations, and would allow Metroid to be released in the summer where gaming droughts might exist.