Sling Media showed off a version of SlingPlayer running on an iPhone just about a year ago and we still do not have a SlingPlayer Mobile client available to install. At first The Boy Genius Report stated that AT&T may have asked Apple to reject the application, but their sources then stated that Sling has not yet heard anything from Apple. So at this time, there still may be a SlingPlayer Mobile for iPhone application coming down the pike. There has also been a bit of controversy surrounding the news that SlingPlayer on the iPhone will only work with the latest and greatest Slingbox products so many people will have to upgrade their Slingbox just to use the mobile application.
With the recent network issues AT&T has been having, such as their failure at South by Southwest, it does seem to make sense that AT&T would be worried about millions of people streaming video content over their 3G network to their iPhones.
You can always use the Windows Mobile or BlackBerry SlingPlayer Mobile clients with your home Slingbox device if you don't want to wait and see if an iPhone application is ever released.
I have an original Slingbox Classic, but have moved onto a Hava player because of their FREE mobile clients for S60, Windows Mobile, and Nokia Internet Tablets. I have way too many mobile phones to buy several copies of the SlingPlayer Mobile client, even if the quality and functions may be better than my Hava. I understand that Hava is also working on Android and iPhone applications so I may never upgrade or go back to using my Slingbox.