Short for In Before The Lock (alternately, I Beat The Lock). Somebody on a Message Board has made a thread that is just guaranteed to get killed off before it goes anywhere. It's inevitable. So, as a rather thinly-veiled attempt by other uses to boost their Post Counts, they simply post IBTL in the thread as it comes up. This is often looked upon with the same disdain as contentless First Post spam, but since most threads that get this treatment are doomed anyway, it's less likely any action will be taken.
If the thread in question gets deleted, then their Post Count spamming is all for naught.