3% of land surface is covered by urban areas according to GRUMP datasets combined with satellite images [1]. Satellite images helped land-use researchers at UW-Madison's Center for Sustainability and Global Environment to conclude that over 40% of earth's land is given over to agriculture [2]. So considering human inhabited land to be a combination of agricultural and urban areas, we have around 43% of earth's land surface covered by humans (There might be overlapping of agricultural and urban areas in the assumption because of inland agriculture, but it should be safe to assume it to be well below 3% since not many countries practice intensive farming and aome urban areas were left dim in the satellite image) (Both 2005 data)
Another interesting perspective was given by a new global map of European Commission's Joint Research Center which was published in the World Bank's World Development report in 2009.[3] According to the report, 95% of world's population is concentrated on just 10% of world's land surface. While only 10% of the world's land is classified as remote or more than 48 hours from a large city. So we can either state that most humans now inhabit only 10% 0f world's land, but we have the rest 80% well connected with roads, highways, farmlands.etc. So depending on time taken to travel, we can say that humans now inhabit 90% of world's land leaving only 10% for wilderness.

Tobler et al. partitioned 217 countries into a total of 19,032 polygons with 295,000 persons per polygon. This amounted to an area of 25.4% of earth's surface. So that would be around 96% being considered as occupied at a population density of 42.45 people/sq.km (But the quadrangles would also include a lot of protected and unprotected wilderness and uninhabited deserts. So it would be far to smoothed to give a proper idea of how much land is inhabited)
Tobler et al. (1995)The Global Demography Project (Natl. Center for Geographic Information and Analysis,Univ. California, Santa Barbara) , Tech. Rep. 95–6.

THIS IS NOT MINE IT IS Kaushik Parashar writing

here are some websites that help

[1] http://www.earth.columbia.edu/ne...
[2]http://news.nationalgeographic.c...
[3] http://www.sciencedaily.com/rele...