form action attribute + url rewriting

[grimaceofdespair]

There's a lot to find on the Internet about both the form attribute
and url rewriting. It all leaves me with one question though.

I'm using a combination of Helicon IsapiRewrite and UrlRewriting.Net.

Example:
[1] Browser [News/] --> Isapi [News/Default.aspx] --|ASP.NET border|--

UrlRewriting [Default.aspx] --> Page Processing

This will give me the dreaded Invalid Viewstate when posting back. I
am able to solve that in several ways. But what does work out-of-the
box, is:

[2] Browser [News/Default.aspx] --> Isapi [News/Default.aspx] --|
ASP.NET border|--> UrlRewriting [Default.aspx] --> Page Processing

Now, I was wondering what actually happens when there's no real
rewriting, but IIS just presents a default document:

[3] Browser [/] --> IIS [/Default.aspx] --|ASP.NET border|--> Page
Processing

To me, this actually looks a lot like my first example, with the url
rewriting. Does anyone have an idea what could be the difference on
the ASP.NET border that would have [3] work, but not [1]? Both have a
client url without aspx and a form action with "Default.aspx".