I’m getting this error too… I did set the index.php permission to 755 but still getting the same error.
When I type this, I get redirected to http://shop.kltinformatique.ca/magento/downloader/
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, firstname.lastname@example.org and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
EDIT: Just noticed this thread is for 1.1.7 and not 1.7 which just came out. Where is the thread for 1.7 support?