The HP MediaSmart Server iStream app [Free - iTunes link] allows iPhone and iPod touch users to stream media from an EX485 or EX487 HP MediaSmart Home Server.
Supported media includes photos, music, and video (though HP says DRM'd video might not stream properly). Part of a much more impressive server package, Engadget has the details:
Dual Core processor, 2GB of RAM and up to four 2TB drives [...] The new EX490 and EX495 can handle up to 7TB of internal storage and up to 17TB pairing up internal drives with external hanger-ons. More interesting, however, are the new software capabilities built on top of Windows Home Server. Mac users now get admin features, media collection and bare metal recovery over Time Machine, and the servers have a built-in video converter for collecting and converting videos for a specific device.
They function as iTunes servers, with the Celeron bound EX490 starting at $549 for 1TB, the dual-core EX495 starting at $699 for 1.5TB.
By contrast, while Apple's own Time Capsule has been rumored to be getting iPhone streaming functionality, nothing has thus far appeared.
If you try out HP's streaming app, let us know how it works for you!