Integrating a .NET Compact Framework application with a Java back end...

From the .NET Developer's Journal (web site)

Connecting Microsoft Mobile Devices to Java Infrastructures
Integrating a .NET Compact Framework application with a Java back end
by Christian Donner

'Summary
If the task at hand is to connect a Pocket PC running the .NET Compact Framework to a Java back end, and if Web services are ruled out as an interoperability solution, there are not many viable options available. The one presented in this article may well be the only one.'

Read the article.