Whilst a modern PDA is powerful, with the fastest at the time of writing (June 2006) having CPUs running at over 600 MHz, Wolfram Research have not announced any plans to release a version of Mathematica which will run on a PDA. However, using WITM on a UNIX/Linux server via a WiF, Bluetooth or other network it is possible to use Mathematica from a PDA. In my opinion, Mathematica using WITM is considerably easier to use than a Telnet or SSH client to access the server from the PDA. This is especially so if the output of the commands is graphics - i.e. a graph or similar. The following 2 images show Mathematica being run first with WITM, then with a commerical SSH client.
Mathematica on a Pocket PC using the free GPL'ed WITM
Mathematica on an hp iPAQ PDA using a commerical SSH client (PocketTTY).
It should be obvious the WITM interface is much better, but this is even more obvious on more complex graphs.