Hi Kelsangnorbu,
I think you should check out www.loosestitch.com. It is a free, online outliner that we have been building. It is still in Beta (version 1.0 should be out soon) so we would really like to get any thoughts you and others might have. Since we have written it using AJAX, we are really excited about getting it to work on the iPhone. We should have something ready in a couple of weeks. As we don't own an iPhone, and can't until late next year (we are in Asia) anyone willing to give us a helping hand to test it on an iPhone would be great.

We also allow importing an OPML document from Omni Outliner. In fact, we feel LooseStitch is a great tool of OO users as it makes working with others on an outline really easy. You can easily invite friends to view/edit your document, publish your outline on your website so that others can view it, or import it back into OO once you are done. This is a great way to share your OO outliners with a Windows user.