I think some people have got a wrong idea of a lookup table. It is not an entity per se. It holds discrete values and their meanings that are generally used for populating screens and reports, i.e. expanding on codes used within the system, where the codes may be "meaningful" to a few people but mumbo-jumbo to the rest of the populace. e.g. CustomerType=1 may mean something to the cheque-clearance personnel at the bank back-office, but be utterly meaningless to the customer himself.

Therefore, in line with Paul, all a lookup table really needs is varchar.