I had the same problem, I had the trial version of blend, uninstalled and then installed Expression Studio (from my MSDN subscription). The option in Silverlight projects was gone.
I found here that all I needed to do was to add the following registry entry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Expression\Blend\VS\BlendLaunchPath
And this should point to the full path where expession blend is (in my case: C:\Program Files\Microsoft Expression\Blend 3\Blend.exe)
After this all I needed to do was to restart Visual Studio and there it was again!
Hope this helps.