Hi there!!! I'm just doing this topic to advice if anyone has troubles after upgrading to Silverlight 5. Today I did it, but suddenly IE9 and FF9 stopped working with hotmail... after trying to look for a solution I had to revert back to version 4... at least for me worked properly. If you experience this problem, do it as well
The bug bassically is that Silverlight 5 just refuses to load attachment tools in hotmail while writting a message, so you are unable to attach something in IE9, in Firefox, it just fails and then, the normal tools are loaded instead.
I hope Microsoft can fix this issue sinse it affects their own browser and others as well...