SlingPlayer has been given the green light to arrive on the iPhone tomorrow (Wednesday May 13, 2009). However 3G access for the player has been disabled so it will only work on WiFi. Clearly AT&T or Apple has an issue with massive bandwidth consumption.
According to Engadget, the app works is good on WiFi but still tends to lag. When in the letterbox mode, there are black bars around the four sides of the image. The application should be available after 12AM tonight. The cost of the application is roughly $30.