As Oak has said, the seed is used to seed a pseudo random number generator, which makes it (in practice) impossible to determine. However, to make things worse, the given seed is first run through a hash function ( http://en.wikipedia.org/wiki/Hash_function ) that (should) generate wildly different values with the changing of 1 letter.
In short, there is no useful correlation between the seed input ("Gargamel" for example) and the seed used (some number). And even if there was it wouldn't get you very far.
Update (for clarification):
This does not mean that using the same seed will generate a different map (else sites like minecraftseeds.info wouldn't work), the hash function will always generate the same output given the same input and the pseudo random number generator will always create the same sequence from this number. It does mean however, that changing one bit of the seed (say, "Gargamel" versus "Gbrgamel") will create a different map, which will usually have very little in common with the first map.