ArcPad currently will not run on an iPad or other iOS device, or Android or Windows Phone OS. The software will only run on a Windows desktop OS (XP, 7, etc.) or on a mobile device running Windows Mobile 6.x. There are several tablets that run Windows, like the Panasonic Toughbook tablets and the Trimble Yuma, but these devices are really highly modified laptops and do not offer the same user experience as the iPad or Android tablets.
You can get ArcPad-like functionality on the iPad using feature services accessible through ArcGIS Online, but the functionality requires either ArcGIS Server or an ArcGIS for Orgs account. Going this route also requires an 'always on' internet connection - no disconnected editing (yet).