Is it possible to deleted the "unbreakable" blocks on single player?
I ask this because of a video I just watched in which 50+ mobs fell down a bottomless pit (i.e the "unbreakable" wasn't there)
If it's possible, it could be a welcome addition to my castle.
No this isn't possible anymore. In the old version of Minecraft there was no ocean and no ground, you could fall off the edge or down the earth and fall forever. Now that Notch placed the barriers you can't leave the world anymore.