I just found on the application CD a free product called Booster.Net. I have to try this, because it let me using some VB.Net developments on my P900. cool :-)
Q: Do Crossfire applications require .NET Compact Framework? A: No. While Crossfire applications are built with Microsoft VB.Net, the Crossfire translation process converts Microsoft Intermediate Language into AppForge byte code in order to run within the AppForge Booster (Device Definitions) environment.
Q: Why do you need AppForge Crossfire and Microsoft Visual Studio .Net to develop applications for the Sony Ericsson P800/P900? A: Microsoft only provides .NET Compact Framework for a subset of Microsoft mobile devices. AppForge provides the only cross-platform framework that enables Visual Studio.NET applications to run on non-Microsoft devices. Crossfire takes advantage of the rich development environment and language support available in Visual Studio .NET to provide the best development experience for all mobile application developers.