Oops, I forgot about this question...
I forgot to mention that I use VirtualBox 4.2.6.
Let me tell you what I did as the behaviour is inconsistent (which is why I asked the question in the first place):
- I boot the guest system without a DVD in the drive and allow the guest system access to the drive
- I insert the DVD:
- the host system recognises the DVD, but I can't access it: the file explorer shows a DVD-symbol, but the name of the DVD doesn't appear
- the guest system allows full access
- I eject the DVD and the DVD is gone on both systems
- I insert the DVD again and get the same result as above
- I eject the DVD and again the DVD is gone on both systems
- now I disallow and immediately allow the guest system access to the drive on the host system
- I insert the DVD again and get the same result as above
- I eject the DVD and again the DVD is gone on both systems
- now I only disallow the guest system access to the drive on the host system
- I insert the DVD again and only the host system recognises the DVD with full access
- I eject the DVD and again the DVD is gone on both systems
- now I allow the guest system access to the drive on the host system
- I insert the DVD again and both system behave as in step 2.
In other words as long as there is no DVD in the drive when settings are changed the guest system takes control over the DVD whenever it is allowed to access it. This is a cool auto-feature.
Now:
- with the DVD still in the drive I disallow the guest system access:
- the host system now has full access to the DVD
- the DVD is gone on the guest system
- I allow the guest system to access the drive again:
- both systems now have full access to the DVD (oops?)
- I eject the DVD and the DVD is gone on both systems
- I insert the DVD again and both system behave as in step 2.
I guess it's obvious where my confusion comes from.
Conclusion: In case you don't mess around with the drive control the guest system grabs control of the DVD when it's allowed to have access to it.