I thought the blood was too terrible looking for it to be from Nintendo, too. Leaving aside the argument over whether Nintendo would make a blood splattery game, you know that they'd make it look pretty good if they did. That IGN video looked like it was made by someone who'd just read The Complete Idiot's Guide to Particle Effects.
They'll never top that Zelda movie trailer. They might as well just stop.
There were also many elements that gave away the fact that it was a lame April Fool's joke...
1. If Nintendo announced a service like "Wii Expand" they wouldn't just announce it all of a sudden. They would either find a big outlet (E3, GDC, Tokyo Game Show) to unleash the new or create a big press conference. Nintendo might be unpredictable with their news and announcements, but they always pick a place and a time for them.
2. If an announcement is to be made you can bet that Iwata, Reggie, Cammie or any important person at Nintendo will deliver it. The video showed a random Japanese woman making the announcement. Again, Nintendo likes to deliver the news themselves and not hire a third party to do it.
3. Why would anyone secretly tape a conference if they were invited to the event? I know that some conferences don't allow for the event to be taped. But even then the media can professionally record a conference without the need of guerilla tactics.
4. The room clearly looked like a regular room with the lights turned off, a big screen TV in the middle of the room and some people to simulate a crowd. Once again, these news events take place in big convention centers and theaters. The room just looked too small for a press conference.
I feel bad for those that actually believed the news on April Fools day...