Thanks for the additonal feedback.
I really think 4.0 should try to leave 'physical' asp.net pages alone. Here is what happend to me.
I did choose
GOOD - Sitefinity did after update to sp1 of 3.7 show my existing asp.net pages
BAD - it added /de/ or /en to the url
1) Image Pathes broke
2) Worse I had default language set to /de/ and had some /de/ in my existing URL - It seemd on all pages with /de/ in the 'physical' path of the existing asp.net pages - sitefinty would go into an endless loop!
3) If you have an only payment system they may except only payment call for very specific URL which you had to tell them. Now Sitefinity adds /de/ or /en/ to the url wich means the paymentsystem will not work because the request comes form some other url then expected!
Seemd to take care of the above problems.
Any feedback on the problemes describet would be nice.
Alos if you can comment what might be advantages/disadvantages using QueryString vs. PathPrefix