Everything worked fine, my Mac has been successfully updated, but after the first boot, the problems started. Mac OS X 10.5.8 is sometimes completely unable to communicate with my router via Wi-Fi (Asus WL 520-GU), which is my default internet gateway. Communication with my secondary router works (this is actually an Asus WL 520-GC, not GU), but it is useless since I can't access the internet at all (the secondary router is used to amplify the signal).
An temporary solution is to turn off the AirPort and start it again, but I can't do this again and again, every 15-20 minutes. I've surfed the internet and I found that a problem like this was occurred on the previous year, somewhere in September, again, after an update.
I'm now afraid that it will take a while until Apple will fix this, because they are now concentrating on the iPad/iPhone HD/4G/ iPhone OS 4.0, and I have the sensation that Mac OS X is currently not a priority for them. I hope I'm wrong.
If you have the same issues, please don't hesitate to leave a comment to let me know I'm not the only one having these issues !