Tag Mapping is not a new ASP.NET 2.0 feature, but only recently many people discover it.
The use of Tag Mapping allow us to, at parsing time, replace a given control type with other control type. The only constrain is that new control type extends primary control type.
If I use Tag Mapping with Web Controls everything works fine but if, instead, I use Html Controls, like HtmlForm, with tag mapping no mapping is done.
Using Reflector I found no reason for this behavior.