You don't need Spy++ to do this. It just helps so that you can see what you're looking for in the end. Here is the relevant part of the window heirarchy for my IE window that has me replying to this thread:
Caption: "Replying to a Message - Microsoft Internet Explorer" Class: IEFrame
|-->Caption: "" Class: WorkerW
.....|-->Caption: "" Class: ReBarWindow32
..........|-->Caption: "" Class: ComboBoxEx32
...............|-->Caption: "" Class: ComboBox
....................|-->Caption:"" Class: Edit
That edit box is the one that contains the URL. Using FindWindow and FindWindowEx, you can get the handle to that edit box. Then you can just get the text from that edit box.
Of course, there might be an easier way to do this. If you get the Internet Explorer SDK, there is probably an easier way.