By Sidney Urquhart

All over Washington, politicos have been nervously checking their mailboxes for days. Where were the invitations to one of the liveliest annual parties in town? Is the Senator annoyed with us? How have we offended him? Not to worry. The postmark on the green-bordered, red-lettered card tells the whole story: PM 11 DEC. 1991. When William Smith's trial ended late last Wednesday afternoon, fleet-footed Kennedy staffers raced to the post office with preaddressed invites to UNCLE TEDDY'S annual Christmas bash.