I'm using a custom 404 ASP error page to rewrite friendly-style URLs to standard asp ones, and I then use server.transfer to call the actual page.
eg. www.mysite.com/london.htm (which doesn't exist)
www.mysite.com/cities.asp?name=london (which does exist)
My question is this - is there any danger that search engines will recognise this as a 404 not found error and so not bother indexing it, or does server.transfer fully hide the fact?