Upper bound hashrate around the current time : 4,000,000,000 MH/s
If all the miners were 300 MH/s GPU's this would equal 13 million miners [1] that would equal 0.0018% of the world population.[2]
In reality I have pushed those numbers to the absolute unrealistic extreme and because one ASIC can hash a hundred or even a thousand times faster than a GPU the percent of world's population hashing is going to be a lot lower (Also I assume one miner per person pushing the percent up even more)
So its between 0% and 0.0018%
Interestingly if everyone on Earth shared the mining we would each hash at 1/2 MH/s (Half a megahash each)
[1] (4,000,000,000 MH/s / 300 MH/s = 13,333,333.33)
[2] (13,000,000 / 7,100,000,000 = 0.001830985915493)
A couple of mathematical corrections . . .
0.001830985915493 is approximately equal to 0.1831%, not 0.0018%
There is clearly at least one miner, so it must be between 0.0000000141% and 0.1831%