Here’s the announcement from Matt Thomas on the WordPress for iPhone blog:
We’re live! WordPress for iPhone just launched on the App Store. Download it now and get to blogging. We’ll have more information about the project and how you can contribute soon.
If you run into issues while using the app, we’ve got a page for reporting them up now. Before you do, please check out a few frequently asked questions.
This WordPress for iPhone app uses XML-RPC to post to your blog, so if you are using WordPress 2.6 then make sure you have it enabled under the Settings -> Writing section in wp-admin.
Version 1.0 is just the beginning, we’re already planning for new features.