Claiming to be "the only iPhone app that lets you write posts, upload photos, edit pages, and manage comments on your blog from your iPhone or iPod touch," WordPress for iPhone supports both WordPress.com hosted blogs as well as self-hosted WordPress (2.7 or higher) installations.
WordPress for iPhone makes use of the XML-RPC Service, which you must first activate through your blog's control panel. This provides a secure connection; with self-hosted WordPress sites having the option to use an SSL encrypted communication link between the iPhone and the app.
One useful feature is the option of automatically resizing photos to 640×480 for faster uploading — an especially important aid for those on slower Edge network connections. This setting can be specified for each individual post. A thumbnail image is automatically generated as well, and inserted into the post, along with a hyperlink to the full-size image. Photos can be captured directly from the camera at the time of posting, or an archived image may be selected from the iPhone's internal memory.
An active user and developer community is available to provide support and ongoing releases of what some operators may find to be an indispensible production tool. If you're a WordPress user that wants the ability to post whenever you want and from wherever your iPhone has a signal, then WordPress on the iPhone is for you.
You can learn more about the app at iphone.wordpress.org.