I have found three APRS apps for iPhone/iPodTouch using APRS in some form. One is just re-directing to an APRS web site available for every one. Other services in that app need subscriptions, which are free on other web sites! Before you buy an app it is wise to read reviews or find info on the web if possible. An other APRS app is more focused on messaging than maps. The app PocketPacket is the best developed one for the moment as I see it. The screen dumps are from the PocketPacket. The left one is part of the set-up screen. I have here set my position manually. After connection to an APRS server the packet flow can be observed as shown in the right screen dump.