Just came across this post on MSDN Blogs. It says "Signing an unsigned assembly in .NET 2.0". Usually I dont find such things interesting, but gone are those student days and being a professional, there are some "corporate" things to be learnt. So recently I borrowed a book from library "Introducing .NET Framework" by O'Rielly. It was an old book, dated .NET 1.0 beta 2 release!! but still I felt I learnt a lot of new details regarding .NET Framework, especially about the CLR and stuff like that. So I found this one interesting. I have made some notes which I would post it in the future.
For now, take a look at this.
http://blogs.msdn.com/neerajag/archive/2006/10/12/signing-a-existing-net-assembly.aspx