The hash power that the network currently has it would be impossible for any singular entity to gain control of 51%. Could have happened in the early days, but I don't think this is a viable threat any more.
A government could quite easily get a 51% control, or when the next generation of ASICs are made, the companyies who build them could also probably do it.
That said, it would be costy and without profit at the end, so it would either be a government sabotaging Bitcoin, or a terrorist attack.