Though i am not a believer of non standard stuff, but recently was asked this question on IRC on how to launch applications via a hyperlink, it was probably alright to do that in the users case, since this was for an Intranet and everyone had that software installed and were running windows. The call:// link on skype gave me the idea that its got something to do with the windows registry (or atleast on the windows platform).
Found an article on MSDN that talks about this:
http://msdn2.microsoft.com/en-us/library/aa767914.aspx
