Are string.Equals() and == operator really same?

Are they really same? Today, I ran into this problem. Here is the dump from the Immediate Window:

?s 
"Category" 
?tvi.Header 
"Category" 
?s == tvi.Header 
false 
?s.Equals(tvi.Header) 
true 
?s == tvi.Header.ToString() 
true

So, both s and tvi.Header contain "Category", but == returns false and Equals() returns true.

s is defined as string, tvi.Header is actually a WPF TreeViewItem.Header. So, why are they returning different results? I always thought that they were interchangable in C#.

Can anybody explain why this is?