What is a good GTD app with cloud sync on iPhone, iPad, and Mac?

Check Wunderlist by 6Wunderkinder: http://www.6wunderkinder.com/wunderlist/

It's free. Iphone and ipad apps are available on itunes app store.

I found a good, cheap alternative to OmniFocus in ToodleDo. It is browser-based (it doesn't get much more "cloud" than that), but they also have an iOS app for offline use. I've been using it for a few years now and have had little complaints even at their prices (free to a fraction of OmniFocus).

As a sidenote, OmniFocus does support cloud-based syncing. From the product page:

Sync

You can’t accomplish your goals if you don’t have them with you. OmniFocus synchronizes your task database with a server or disk, so that all of your Macs, your iPhone, and your iPod Touch are up-to-date. Sync over your local network using Bonjour, through the cloud using MobileMe or any standard WebDAV server, or even with something as simple as a USB drive.

I believe this is a recent development since their iOS apps. OmniGroup products are not cheap, but they are definitely worth the expense.