A PalmOS based solution that would cover most of my needs
has been released. Priced at only 14 € it offers a lot of
features:

http://www.schlangmann.mynetcologne.de/HarrysLapTimer/

Because of this the project is frozen. The effort of a duplicate solution
wouldn't make sense, especially because there are too many details to work out,
12V supply and TFT/LCD for car use to name some of them.

But when or if EFIKA evolves into a PDA, it will be worth
reconsidering this custom tailored and Linux-based solution.