Unfortunately OmniFocus is a closed, proprietary solution that does not publish an API. This is clearly their business strategy since they have rejected many user requests to open themselves up to the cloud. Omni is an Apple-friendly software company that assumes its customers will use only Apple products (Mac, iPad, iPhone, etc), so their applications (and sync capabilities) are only supported between these platforms.
The closest thing they have to an API is an AppleScript interface that lets you manipulate task data on your desktop. Back in 2007-2008, someone wrote and published a free AppleScript that synced OmniFocus with Toodledo, and it worked great. The problem is that every time Omni releases an update to their software, it will likely change their data structures (including the AppleScript API) and therefore break any scripts that rely on it. This is exactly what happened with an update in 2009.
Another way you could potentially interface with OmniFocus is by reverse engineering their XML via their WebDAV sync. When OmniFocus syncs with a WebDAV server, you can view the text files and see how Omni represents its data structures, so you could potentially exploit this and write your own API. Again, in addition to the extraordinary amount of development work it would take to reverse engineer their data structures, you would also have to face the fact that these structures would change after each software update. So unless you make a full-time job out of this, it's not a feasible solution.
OmniFocus is by far the best application for GTD. It follows David Allen's methodology both literally and conceptually better than anything else I've seen, and the user interface is well-designed and optimized for productivity. However, if you have to spend a significant amount of time managing tasks on any non-Apple platform (i.e., Windows PC, Blackberry, Android, etc), I recommend that you find another GTD product.
I can't blame Omni for not publishing an API...it's their business strategy, and an API could potentially reveal some of the "secrets" that make their products work well. At the same time, however, I don't think it's good for a long-term strategy in a world that is increasingly moving to multi-platform cloud services.