Well, considering that Methane is at least 20 times [from memory its actually much more than 20, possibly as high as 50-200 times] worse than CO2 in terms of global warning - Methane is a big issue for Kyoto protocol signatories.

Methane from farm animals is responsible for over 50% of the Greenhouse emissions emitted by NZ [unlike any other country in the Kyoto protocol].

The NZ Government's position, is that the tax will help part-fund (note part-fund - the government is already paying the rest) research into ways of reducing the methane emissions from farm animals - which if successful, will help NZ meet its Kyoto protocol CO2 emissions earlier than 2012, and may even allow the knowledge to benefit other countries who also raise livestock.

The tax is quite small - a few cents per animal, per year. I think its reasonable.
Other governments would just fund the whole research and bury the cost in the usual government spending "slush funds".

The initial reseach results so far, indicate that quite small changes in diet or soil/gut bacteria "on the farm" can have a big effect in how much Methane is emitted from the animals.