It might be a Windows Mobile issue. When Windows Mobile "creates" a new wireless ad hoc network it mayset it to use WEP with automatic key distribution. Which means that message broadcast won't work. Do you have a laptop with you? Try creating the ad hoc network from the laptop, and then join the network with the PDAs. Just to see if it works and that they see each other. If this works, the next time the PDA is switched on it will use the non-WEP ad hoc network, even if the laptop is not present.
By the way, we just released OpenTrek 3.5 beta 2 which includes Embedded Visual C++ 4.0 sample projects and DEBUG binaries of OpenTrek launcher for stationary PCs. Get it at our download page at www.develant.com
//Johan