Hi Albrecht
thank´s twice.
You're welcome, I face the same problem, so if I can give feedback for user, that's a pleasure, and it also helps me to understand better drupal internals
Well I expect 1000 user per day in November/December.
In fact, the problem may occurs during node/user creation, but if there is not so much I don't think you encouter this issue. And as I told you it's a race DB problem, so it's very very rare for this to happens, but it's better to put the LOCK to avoid this kind of problems. If you want to go deeper, I would suggest to have a look at the post that made jeremy about file-based locking : http://drupal.org/node/21556 it's a little complex but talk about the race problem making DB locks...
Well, hope you will have good time using drupal, feel free to ask more support.
Regards
Nicolas