Now that I'm well on the way to replacing the 2.8L engine with a 3.0L engine and have stripped the old block to sell any usable parts on eBay, I decided to take a look at the hole that started this whole process.
To recap, when I took off the intake manifold I found the valley filled with oil. While cleaning out the gunk, I found a hole through in the block.
This led me to pulling the engine. Upon removing the oil pan I found a chunk of aluminum from the inside of the engine just sitting there. It's not clear if my cleaning broke it completely free, or if it had been there for some time now.
With the block stripped down, I could see the hole clearly from the inside. I could also see the wall on the other side was starting to fail, and would likely have gone soon after.
So there you go. I'm not sure if this is where valley failures commonly occur, or if my block is anomalous.