I know Seed and the Corruption dot overwrite each other. But while running a few instances, I started to notice that my seed was disappearing right after I cast it. I checked recount, and there was never any damage attributed to Seed. I don't remember the exact group makeup, but it was a DK tank, a tree, a dps warrior, and I don't remember the others.
Once I noticed that, I started experimenting. I would cast Seed of Corruption and then do nothing else at all, but the spell would still disappear.
I'm sorry if olde question is olde, but I only play my lock once in a while, so I don't keep up with the class mechanics as well as I should.