Ethiopia has cancelled its Visa on Arrival programme effective Sunday 1 April 2012.
The British Foreign Office posted the following on their travel advice page:
British nationals require a visa to enter Ethiopia. From Sunday 1 April 2012, it will no longer be possible to obtain a visa on arrival at Addis Ababa (Bole) and Dire Dawa International airports. You must obtain a visa prior to arrival at any port of entry, even when travelling from countries without an Ethiopian Embassy or Mission. Visa applicants in the United Kingdom should contact the Embassy of the Federal Democratic Republic of Ethiopia in London.
However, the Ethiopian embassy website does not yet (28 March) state that the VoA scheme has been terminated.
According to one source, visas on arrival will be available until 8 April under emergency transitional arrangements, but travellers who are planning to visit Addis Ababa Bole airport as their first port of call are strongly advised to obtain a visa from their nearest Ethiopian mission (who are able to provide a quick turnaround for visa applications).