QuickMark's 2D barcode reader appears to be working on the iPhone. The short video shows the QuickMark software successfully decoding a QR Code containing a UTF-8 encoded Chinese string. No download is available yet, but we are not supposed to be able to do that on an iPhone anyway (at least not officially).
So far the iPhone is still a supposedly proprietary platform, although several developers have already been able to create applications for jail broken iPhones.
Such actions are probably frowned upon by Apple, so they started a iPhone Developer Program and plans to launch its App Store platform later this year to set things straight (and make more money). But it remains to be seen how many 3rd party applications will be available when the Store goes live and when iPhones start to come with version 2.0 firmware.
In terms of barcode reader applications for iPhone, iMatrix is quite active and is looking to add support for a variety of proprietary barcodes. Google's ZXing project has iPhone on its road map and Neomedia has also released a couple of screenshots.
Although the iPhone has plenty of barcode reader support, it is not equipped with a great camera for reading barcodes. From my brief experience with the iPhone, barcodes will probably need a module size of at least 0.8 mm to have a chance of being recognized by the iPhone (in Japan, QR Codes are sometimes printed with 0.25 mm module sizes).
Update: QuickMark for iPhone is now available on iTunes App Store at: http://tinyurl.com/c3obf5