Hi,

Check out NirvanaHQ. It was built specifically for GTD and the developers seem engaged in making it a close fit with the 'canon'. It is a web-based solution and has a native iOS app. Last I heard they were developing an Android app too.

I don't use it myself but did try it out during the beta and it looks good. It has an active community of users too so there is decent support for it.

www.nirvanahq.com