Suggest you consult the JARs themselves. Surf to:
http://www.jaa.nl/section1/jarsec1.html
and look at JAR-FCL 1 (flight crew liecensing - aeroplane) and FCL 3 (flight crew licensing - medical).
I looked into this recently. As far as I could deduce, the main constraint is that your initial medical must be undertaken in the same JAA state that hosts your training, exams and issues your first licence. Once you have the licence, it can be used interchangeably in all JAA states and you can add endorsements in any of them.
I can't say if it's as 'simple' in practice, can anyone else comment?