If I remember correctly, it is stored separately as an NBT tag associated with the block. you would need either a worldedit filter or NBTedit/other external program to be able to set the spawners. After all, you can make mobs spawn with armor, different weapons, and enchantments on that armor. that won't all fit into a block ID, so the guys at mojang use NBT files instead
I KNOW there is a mod that lets you place any mob spawner, but I can't remember which one it is. And I have used in my local server, so I know it works in SMP. Maybe NEI? I know that what this mod does is showing in the NEI gui a different spawner for every mob, so you can select the one you want and place it in the world. It even works with mobs from other mods. But I can't remember which mod it was... =(
Yes NEI is the mod that lets you do that i had it for a while and i has all the mob spawners for each mob.
This topic has been locked due to continued bumping by users. Please be aware that this thread was started 2011. It is more than 2 years old and does not need any more solutions as the problem is already solved.