How to detect incoming call on N900 and display info window based on caller?

Does the N900 allow me to display additional information in parallel to the native application or does the latter always have priority over my process?

I'm interested in displaying additional information based on caller id.

If it's possible, can you name any pitfalls or give small python code examples / or tipps to get started?

detecting incoming call might be the smallest problem you will see in this journey - you can start with this thread

now consider few other factors before you decided whether you want to continue or not:

1. calls come not only as phone call but also as SIP call, Skype call, GTalk call, etc
2. call signaling is relatively resource-heavy due to time constraints vs blocked by I/O, etc
3. call dialog should work ok in portrait and landscape, so you might need to go down extending call architecture not writing my own little thing in 1-2 weekends
4. internal eMMC storage is not quick and gets slow on 2+ threads trying to write

if you are Ok with risk to spent time and bump into limitations of Maemo5 platform put on end-of-lifecycle hook -- consider learning down googleing keywords maemo5 telepathy mission-control . this is starting point not definitive guide -- you have to learn quit many different things before you start to approach plugging call progress dialogs.