As the main reason why I got Nokia N950 is that I promised to write some applications for that, I started to look deeply at available options how to write them. Unfortunately there are too many options and all documentation I've found is pretty confusing.
The short story is that you should use Qt for user interface. However it is not that easy as it looks, you can use regular Qt, which can be run on the phone, but looks ugly in the end (maybe I did something wrong, but it did look completely different to anything else running in the phone). You can use Qt Quick, which is easy to write (simple UI definition and JavaScript variant for code), but it does not seem to support much of the widgets. And then there is Meego Touch library, which seems like a best option, however not mentioned at all on Nokia developer website.
Looks like I need to learn few more things before actually starting to code.