Just so you know, Missingno isn't a glitch. It's a programming data residue. It's what's left when they take something fromt he game that wasn't meant to be removed. For example, in Red/Blue/Yellow, they removed the data for Mew. That's where Missingno came from there. In Gold/Silver, they didn't change anything to my knowledge, so the only way to encounter a Missingno there was through cheating, in which case, all the codes did was created a small piece of data and placed it into the game program, then deleted it instantaneously. In Crystal, Missingno was quite easy to come across, if you knew what you were doing. They removed a lot of stuff from that game, like the Wireless Tower (whatever it was called), and Celebi. In Ruby and Sapphire, they took out Jirachi (I believe, don't quote me on that part), and Missingno returns again. I don't care what anyone tells you, you can encounter Missingno in Ruby and Sapphire anywhere in the game without having used any cheating devices.
The glitch, as you call it, is just a piece of data left over that the game doesn't recognize after it's been removed. There's a gap. It's like saying something like this: T*** s******* can only be p***** re**. (This sentence can only be partly read.)
The game doesn't recognize whats left, and creates the default image; Missingno.
In FireRed/LeafGreen, there are many new things that we don't know yet, even through the Japanese games. So it's not sure if there'll be another Missingno appearance here, aside from device usage, but I wouldn't doubt it.
(And I still keep thinking Celebi's in the Pattern Bush somewhere. =p)