Determining What Url The User Originally Entered To Visit A Web Page After Redirection?

Currently, I'm just using clientside Javascript (location.href), but I am wondering if there is a way in Asp.Net to figure out the URL the user originally entered (assume I did not change it myself via 301), or at least to track it in a simple and reliable manner. As I am using my own implementation of URL rewriting via the global.asax (e.g. Context.RewritePath), this is not an easy task, particularly since I don't want to touch it too much.

Example

Global.asax:
public override void Init()
{
base.Init();
this.BeginRequest += new EventHandler(Global_BeginRequest);
}
void Global_BeginRequest(object sender, EventArgs e)
{
if (VARIOUSCONDITIONS) Context.RewritePath("SOMEURL");
}
SomePage.aspx.cs
protected void Page_Init(object sender, EventArgs e)
{
//Request.RawUrl is equal to "SOMEURL", as
//are other properties that store the URL.
}