I'll add some that you can look into. All of these are either same iPad simultaneous play or pass and play:
TowerMadness - Tower Defense game. Unlock the levels with single player, then compete to see who can handle the levels better.
Harbor Master - Direct ships in and out of the port. Either compete or work together (Although the co-op seems like little more than two people playing the single player mode, except with two scores that combine). This is a favorite of mine
FlightControl HD - Much like Harbor Master, except with planes and helicopters landing rather than boats getting in and out of the docks. I don't like it as much as Harbor Master, but it's still cool.
Small World - A great board game, if you are into that. It's currently only 2 players, but you can now play with another player (Side-by-side or across the table from each other) or AI. Pretty much the best board game adaptation on the system so far.
Werewolf: Curse of the Pandora - If you have a large group of people and are too tech savvy to use cards, this is a cool version of the classic Werewolf/Mafia "deduction" game.
Fruit Ninja HD - Slice up fruit as it flies through the air with your sword, but avoid the bombs. Or play the zen mode where there are no bombs. Playing against each other allows you to throw bombs at each other occasionally.
Also, the two that I have that have been mentioned before but I think deserves another mention due to how much play time they get with my wife and brother: Marble Mixer and Boggle.
In addition, there are a couple great iPhone/iPod games that still make great iPad games. Roll Through the Ages can support as many players as you want to add. It's a dice rolling game. Simple but very fun. Carcassonne is a great adaptation of the tile laying board game. If you've played it, you probably think it's a blast. If not, do yourself a favor and try it out. Both of those games are supposedly getting free upgrades to iPad versions in the future. But you may want to confirm that if it's important to you. Also, Zooloretto is fun. I haven't heard anything about an iPad version, but it would be perfect for it. It's another Board game adaptation where you are trying to build up your zoo by bringing in the right animals to fill your habitats, and sticking concessions outside the right pens in order to make more money off the best attractions you have. Quite fun. 3 players initially, with players 4 and 5 unlockable by playing the game. Both Carcassonne and Zooloretto allow any mix of real players and AI players. So that's cool.
Also, if you find these games fun, you'll want to keep an eye out for Neuroshima Hex, which should be close to releasing it's iPhone version very soon, with the iPad version lingering sometime behind that. However, the iPad version will not be universal, so you will want to hold off until that version is done unless you just don't mind buying a non-optimized version.
Anyway, hope this helps. I've gotten over 24 hours of gaming from my iPad in the week I've owned it, so I understand your needs exactly. Good luck!
