OK, it could have been so easy by just trying it out:

With Process.uid a script can change and read it's uid as it likes to.
If ruby has uid=0, there are no boreders. The script gets up and down 
to 0 again. Else the script is forbidden to change to 0

Robert K. schrieb:

> Hi,
> can I somehow change a ruby interpreters uid from 0 to something and 
> then back again?
> 
> I intend to write a little program, wich traverses all users dirs and 
> 'executes' whatever a user has written in his conf-file but in the 
> context of the corresponding user.
> 
> What is the ruby mehtod to accomplish that?
> 
> Reinvoking my script?