I had the same problem, I had a trial version of the blend, uninstalled and then installed Expression Studio (from my MSDN subscription). The option in Silverlight projects has disappeared.
, , , :
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\\Blend,\VS\BlendLaunchPath
, ( : C:\Program Files\Microsoft Expression\Blend 3\Blend.exe)
, , Visual Studio !
, .