No one as of today is rich enough to create create enough ASICs to control 51% of the network. Moreover in fact you need a bit more than that.
A Government could do it quite easily. Or as someone pointed out earlier in the thread, the Chinese government could march in to mines in its country and take over more than 75% of the hashrate anyway, which is a risk with bitcoin.
My view is that most Governments are simply not interested enough to do this.