That essentially means "yo, guys, I'm almost done with hell world, it'll be finished up today a'ight."
Or, in cultured gentleman speak, "Good morning, my friends. This fine day I'm most pleased to announce that I have, in fact, completely much work on the Hell world and will most definitely finish it up later this particular day."
Im pretty sure the new block is either a main part of hell or a new sheep block... Or a HELL SHEEP!!! (What if one of the new mobs are Creeper sheep? You hit them and they give you creeper colored wool but if you arent careful they will explode...
Red-eyed sheep of eternal suffering and perpetually flaming wool. Their endless screams of sheepy agony can be heard from miles away. Some say they scream with the voices of a thousand damned children.
I'm in two minds about the whole Hell thing. As a sandbox-style player rather than a hardcore-scares style player, I find Hell to be of dubious value to that playstyle.
It seems like a very good way to get killed and lose all the stuff you're carrying, permanently. The 10x fast travel seems like an excellent way to get lost a very long way from your spawn point.
It also seems like a very cool place to discover/visit and I'm betting Notch will put a very handy resource there and there alone, to tempt bold players to enter. But see the Getting Killed Very Easily And Unrecoverably point above.
Or - here's a thought - is Notch going to completely change the death mechanic?