Baburam Bhattarai (left), Vice-Chairman of the Unified Communist party of Nepal (Maoist) has become Prime Minister with the support of the United Democratic Madhesi Front. Together (plus smaller allies) they gathered some 340 votes for Bhattarai, slightly short of an absolute majority in the 594 seats' Parliament but enough to secure the appointment of the communist leader as Prime Minister.
The other candidate, Ram Chandra Poudel, of the Nepali Congress Party got only 235 votes, even if it gathered the support of the Communist Party of Nepal (Unified Marxist-Leninist).
The UCPN(M) alone has 230 seats, more than the Congress and the CPN(UML) together.
According to Harry Sims, posting at Revolution in South Asia today, the Congress party and their Marxist-Leninist allies reject Maoist participation in government because it retains a separate military apparatus (negotiations to integrate it in the Nepali Army have repeatedly failed). Also that:
The Maoist victory was made possible, thanks to a last minute ‘four-point agreement’ with the Madhesi parties on fundamental issues of ‘peace, constitution and functioning of a joint government’. The Maoists agreed to ‘positively revise’ their proposal for the integration and rehabilitation of Maoist combatants.
Political sources told The Hindu that the Madhesi parties would get 10-12 ministries, including Home and Defence.
See also: Wikipedia: Politics of Nepal.