sirena, do your have to set manually on google every url you put ads on? If not you should not fear this. I don't know if you understood how this works, but the pages are the exact same, only the url changes. The diference here is: you will get your google visitors on your same old pages but they will reach then through a different url, that's all.
I actually use this mode in another way, instead of poiting the rewriten urls to my viewtopic.php i point them to viewprintable.php, another mod out there. This way, the users that arrive from google will arrive to the topic printable page. If you do this you can put ads on those pages and keep your original topicview pages ads clean so you keep your regular users from seeing ads but you still show them to your google visitors. It depends on what you want to achieve.
glucarelli, there is no way extended characters are causing any trouble because they simply are token away. Including them in the urls is not recomended at all and can cause you a lot of trouble. If you still want to include them i would sugest you replace 'é' by 'e', 'ç' by 'c', etc. You can make this replacement in the fucntion clean_url() on my mod.
You can change the rewriting to the format you pointed, but you will have to go through the mod and change it yourself. You need to adapt the regex both in your the function clean_url() and in your .htaccess and you need to twaek the link output in lines:
107,88 and 141.