With the latest release if you create a folder somefolder and put a empty text file there named Default.aspx it will work using the url www.mysite.com/somefolder
Other than that its not possible unless you get an add on for IIS. The problem is by default .NET only handles requests for certain extensions like .aspx, images and other files like .js are handled directly by IIS without invoking .NET code at all.
So the problem with www.mysite.com/somefolder is that IIS will get that request and never hand it off to .NET so there is no way (without add ons for IIS) you can do anything like concating the default.aspx because your code won't run unless IIS gives the request to .NET for handling and it won't do that unless the request is for an extension that .NET handles. So by having a real folder and a real file Default.aspx then since Default.aspx is a default document IIS will hand the request to .NET code for handling.
Hope it helps,