ArcPad only runs on mobile devices with a touch-screen and a supported OS. See the link in the Related Information section below for ArcPad specifications.
There are currently two different types of Windows Mobile 5 devices: the first type is WM5 Pocket PC Edition (WM5-PPC), which typically run on larger phones that often have a touch-screen. The second type is Windows Mobile for Smartphones devices. ArcPad is supported on the WM5-PPC but does not run on the WM for Smartphones OS as this OS does not support touchscreens.
All Palm based Smartphones have touchscreens but ArcPad is not supported on the Palm OS.
ESRI does not support a particular device and has not tested on all mobile devices due to the increasing number of mobile devices being produced.