[Phanx]

ItemRefTooltip:SetScript("OnTooltipSetItem", function(self, ...)
	if IsShiftKeyDown() then
		GameTooltip_ShowCompareItem(self, 1)
		self.comparing = true
	else
		self.comparing = nil
	end
end)

According to the code in tekKompare, you also need to have some extra hooks to prevent issues, since the ItemRefTooltip isn't really designed to work with compare tooltips:

ItemRefTooltip:SetScript("OnEnter", nil)
ItemRefTooltip:SetScript("OnLeave", nil)
ItemRefTooltip:SetScript("OnDragStart", function(self)
	ItemRefShoppingTooltip1:Hide()
	ItemRefShoppingTooltip2:Hide()
	ItemRefShoppingTooltip3:Hide()
	self:StartMoving()
end)
ItemRefTooltip:SetScript("OnDragStop", function(self)
	self:StopMovingOrSizing()
	ValidateFramePosition(self)
	if self.comparing then
		GameTooltip_ShowCompareItem(self, 1)
	end
end)