The second format of the Invitational is Block Party. Each competitor gets to choose a stand-alone block and then play a deck that's legal in that block. From Ice Age block, the obvious deck is Necro. There aren't a lot of support cards, but Abyssal Specter, Icequake, Soul Burn, and Mind Warp aren't terrible. Plus, Knight of Stromgald, Contagion and Lake of the Dead are quite good. Back in the day, Thawing Glaciers was legal in block-constructed and thus Counter-Post was an excellent deck, but with thawing Glaciers banned, Necro seems to be the only deck from that block being considered.
The next block is considered by most to be the weakest. The top Mirage-block deck was Ertog. Several people thought about Sandsipoise -- a personal favorite of mine -- but it's only good in a creature-heavy environment. With two different Urza-block combo decks in the mix, Sandsipoise isn't as good a choice. Rath block is one of the stronger blocks with Living Death, Tradewind/Awakening, Hatred, and Counter-Phoenix all worth putting together and trying out. Counter-Phoenix isn't very good against Abyssal Specters or against the fast mana from Urza's block, but I expect the other three decks to be quite popular. I also won't be at all surprised if Brian Hacker trots out his trusty Humility/Prayer deck.
Urza's Block is both fresh in everyone's mind and pretty powerful. Even after all the bannings, combo is still pretty good in the form of either Replenish or Bargain. Rumor had it that both Jon Finkel and Steve-O were planning to play Bargain, but Finkel seems to be questioning his decision and considering an audible after Dave Price beat him 9 out of 10 games with an Ice Age-block Necro deck. Squirrel-Prison and Suicide Black are both live options from Urza's block.
Masques block is legal, but it seems unlikely that anyone will choose to use it. With just two sets released so far (Mercadian Masques and Nemesis), the block doesn't have as many cards as its rivals. Perhaps more importantly, the next Pro Tour is a Masques-Nemesis block-constructed tournament so if anyone did come up with a really good deck, they would probably save it for the Pro Tour.
Block Party seems like a quite interesting format and I look forward to finding out what 16 decks get played and also what wins. I'm sure there will still be speculation about what to play long after the Invitational is over.