MQSeries Everyplace provides an interesting lesson (Score 1)

Take a look at:

http://www.ibm.com/software/ts/mqseries/everyplace

This is a variant of the MQSeries enterprise
messaging product aimed at pervasive devices.
It is optimised for security and size (both
in terms of message size and footprint
on the device).

The interesting observation is that although
this is a fully Java-based implementation, IBM
has produced a C-programming language version
of MQe targetted at Palm devices (for example).

In answer to the original poster's question,
I would suggest that there is a market for
Java-based applications on PDAs but if you want
success on all platforms, you need to take into
account the support for - and the performance of -
Java on some of the lower-end machines and
design your implementation to support a C version
if customer demand requires it.