Update Query With Parameters

I have a web form that is populated from the DB values. User can update teh record by keying in new values in the form fields. In my .vb class file I have an update statement -

strSql = "UPDATE msi_Persons SET Firstname=@firstname where Id=@id"

 I have the parameters defined as

cmd.Parameters.Add(New SqlParameter("@firstname", _FirstName))

cmd.Parameters.Add(New SqlParameter("@id", _Id))

 _FirstName & _Id are the private variables whose values a set thru set and get methods.  I expect _FirstName to have the new value I am keying in the form.

Can any body help me with what's wrong here? This is not updating the database. When I do trace.write it shows me the Update statement as "UPDATE msi_Persons SET Firstname=@firstname where Id=@id" instead of @firstname and @id being replace by actual values.

Please help.

Thanks,
Shaly