Actually, there is another way to get a Mew, but it's only for Diamond/Pearl/Platinum, and it was intentional, unlike the aforementioned glitch.

You need to own My Pokémon Ranch. Upload 999 Pokémon, then the ranch proprietor, Hayley, will have a Mew. If you bring her an egg (doesn't matter what's in it), she will offer to trade her Mew for the egg. You can then withdraw the Pokémon for usage in your game.

As for the glitch above, I can certainly vouch that it works. I have one Mew from Pokémon Blue currently in Pokémon Silver.