A 1% opacity is required. In the case of invisible buttons (buttons which contain only one solid layer at 1% opacity) this is not enough however. This type of graphic must be saved as a PNG-24 file in Photoshop, otherwise the resulting button will not be clickable once downloading to the remote even though it is in the Simulator. I'd consider this a bug of sorts. PEP downloads the graphics to the remote in some sort of optimized format. Clearly during this process these specific types of graphics don't translate properly if they were saved at a lower color depth. So long as they are saved as PNG-24 files you're good to go thankfully.