[Phanx]

In the main chunk:

local dropdown = CreateFrame("Frame", "MyAddOnUnitDropDownMenu", UIParent, "UIDropDownMenuTemplate")

UIDropDownMenu_Initialize(dropdown, function(self)
	local unit = self:GetParent().unit
	if not unit then return end

	local menu, name, id
	if UnitIsUnit(unit, "player") then
		menu = "SELF"
	elseif UnitIsUnit(unit, "vehicle") then
		menu = "VEHICLE"
	elseif UnitIsUnit(unit, "pet") then
		menu = "PET"
	elseif UnitIsPlayer(unit) then
		id = UnitInRaid(unit)
		if id then
			menu = "RAID_PLAYER"
			name = GetRaidRosterInfo(id)
		elseif UnitInParty(unit) then
			menu = "PARTY"
		else
			menu = "PLAYER"
		end
	else
		menu = "TARGET"
		name = RAID_TARGET_ICON
	end
	if menu then
		UnitPopup_ShowMenu(self, menu, unit, name, id)
	end
end, "MENU")

local menu = function(self)
	dropdown:SetParent(self)
	ToggleDropDownMenu(1, nil, dropdown, "cursor", 0, 0)
end

And in the function where your frame is created/configured (your layout's spawn function if you're using oUF):

	frame.menu = menu
	frame:SetAttribute("*type2", "menu")