Cookieless Session Support in EPiServer
As in "No - we don't support cookieless sessions when using Friendly URLs". Your natural reaction is then probably to ask "Why not?". The basic design guidelines for our software is that we should build upon and extend the .NET / ASP.NET framework and that would imply that we should support cookieless sessions (CS from here on).
If you have never heard of CS before you probably want to check this article for some background information. http://msdn.microsoft.com/en-us/library/aa479314.aspx
As hinted in the article, the implementation of CS in ASP.NET is somewhat fragile in itself. There are a couple of guidelines that you have to follow in order for your site to work properly with CS. Basically you need to make sure that all yours that you put in the HTML response are relative or you need to call a special method (HttpResponse.ApplyAppPathModifier) to include the CS URL segment.
Another aspect of CS that is far from ideal is the fact that it is extremely easy to hijack an existing session - you simply need to copy the URL. Cookies are here to stay and my personal opinion is that CS is a leftover from the old "cookies are evil" debate.
Back to the techie stuff - why does this not work with FURL? All links that we generate from permanent links could be adapted to call ApplyAppPathModifier if we detect that CS is enabled. Unfortunately there is another thing that will break the solution. If you generate links in the standard ASP.NET way and that involves the System.Web.UI.Control.ResolveUrl method, you will implicitly call ApplyAppPathModifier and write the CS URL segment.
This is bad since there is no way for the FURL module to reliably detect the CS URL segment in a URL (short of doing nasty reflection tricks) and we need to do that in order to properly rebase all URLs when we do FURL rewriting.
To summarize: In order to support the somewhat brittle CS system we would need to introduce even more restrictions. Since there has been practically no requests for EPiServer to support CS we have decided to say "CS is unsupported" for the time being.