Withdrawing an offer, whether verbal or written, after it is accepted is a breach of contract. Offers of employment can be made subject to certain conditions, eg satisfactory references, in which case the contract will not be complete until that condition is satisfied, however if they have not applied for references this is not a valuable reason.
I think take this further Kirsty, The offer was withdrawn and this has had nothing to do with anything like an unsatisfactory reference. Whilst a verbal reference is not worth the paper it isnt printed on i think you can have fun with them a little. It is very rare for a company to withdraw an offer and so i think you should complain strongly.