Having Mono 2.6 released what is the current status on scalability , portability of ASP.NET application and desktop application on mono.
I would say that the major thing in this release is LINQ to SQL support, however more complex queries won't work so the main area which is still not fully compatible is this. There are still many ways how to solve (or workaround) this issue either by plain ADO.NET (MySQL, Postgre SQL and SQL Server works perfect) or NHibernate (didn't test it but I saw somewhere that it works). Entity Framework is in plans for Mono as far as I know.
I was working (for about 4 months) mainly with ASP.NET and ASP.NET MVC apps and there were no major issues. I'm using Visual Studio for development and then I deploy apps to Linux machine (OS - Ubuntu, web server - Nginx). Even the initial support for C# 4.0 like dynamic keyword works. That's my experience with Mono for now.