Palm’s Treo 750 is starting to show signs of age – a successor definitely is due (especially when looking at the recent price drops). An insider inside of Palm’s developer program has informed TamsPPC that the company has invited developers to submit their applications to be considered for inclusion into the launch program of two upcoming devices called Zeppelin and Skywriter. Additionally, a mock-up image of one of the two devices (showing the physical layout) has been included in a PowerPoint presentation. You can check out the mock-up image and invitation sent to developers here, at TamsPPC.