Missingno. isn't a glitch. Missingno., M-block and all those other block Pokémon were created purely for filling up the hex coding. There needed to be 256 sprites, while there were only 151 Pokémon, about 25 trainers and 3 other random sprites (ghost, Kabutops skeleton, Aerodactyl skeleton). You do the math. Missingno. was man-made, not glitch-made, because:
1. it has stats, which are the same in every copy of the game
2. it has a sprite, which are the same in every copy of the game
3. it has attacks, which are the same in every copy of the game
4. it is able to learn attacks, through TM's, HM's and levelling up, which are the same in every copy of the game
5. it has evolutions, which are the same in every copy of the game
6. it has a name. Its name aren't some random patches of sprites, it is Missingno., which is an abrivication for Missing Number. That means that the people who created him knew at least knew about Missingno. and made a name for him.
Those things couldn't have existed if it was simply a glitch. It was a real Pokémon, just a very strange one. Like Mr Delcatty said, Missingno. isn't a glitch, the way to find him is.
And I think Missingno. glitches up your game so the people who created the games would make people stop using cheating devices to stop people from hacking into the game. Kinda simular to Bad Egg now.
Although it would be fun to have an Easter Egg that at least hinted towards Missingno., I don't think it will happen. At least not as obvious as you people suggest.
And isn't there already an unbelievably small chance of finding Missingno. in the wild on some routes in R/S? Or was that never proven, like the Celebi thing? If they did that on purpose, I think that could qualify as an Easter Egg.