How can I reliably get the actual URL, even when there are percent-encoded parts in the path?

IIS and ASP.NET (MVC) has some glitches when working with urls with %-encoding in the path (not the query-string; the query-string is fine). How can I get around this? i.e. how can I get the actual URL that was requested?

For example, if I navigate to /x%3Fa%3Db and (separately) to /x?a=b - both of them report the .Request.Url as /x?a=b - because the encoded data in the path is reported incorrectly.