By the time you read this post (or 12 PM EST, whichever comes first), Cows vs. Aliens should be live on the App Store and ready for purchase. I first got to see the game back at Macworld (where we also talked about XMG Studios' comments on EA's big holiday sale), and as you can see from that video, the game is a pretty simple affair. It's completely touch-controlled, and the idea is that you push cows into the barn to keep them away from aliens, who you try to push off the precariously situated cliffs on either side. The gameplay comes in waves, and I believe there's only the one mode, but there is Game Center integration for leaderboards.
Cows vs. Aliens seems like good fun, especially for younger players. If you want something more complicated, there's plenty of that on the App Store, but if you want something for the kids to play (or something you can play with them -- the iPhone version works great on the iPad, and the game supports all 11 points of touch, so multiple people can play at the same time), Cows vs. Aliens seems like a good deal. It's US$0.99 on the App Store now