Forgot to mention I tried that. It ran, but not without multiple item conflicts, eg:
10:46:36 [iNFO] CONFLICT @ -204 item slot already occupied by net.minecraft.server.ItemWithAuxData@16ea269 while adding net.minecraft.server.ItemMobSpawner@68cb6b
That's exactly the error I'm having. Running on a headless Ubuntu VPS, Java 1.6.
I've redownloaded the zip a few times, ran it as root; nothing seems to work. I'm starting to suspect the files in the zip might actually be corrupted, but that's unlikely considering everyone else isn't complaining.