As union membership falls, inequality rises. Fact.

Professor Colin Gordon, writing on the Economic Policy Institute blog, charts the impact of declining union membership on income inequality in the US and finds that declining unionisation accounts 'for about one third of the increase in equality in the 1980s and 1990s':
http://www.epi.org/blog/union-decline-rising-inequality-charts/

Gordon asserts that there is a ‘demonstrable wage premium for union workers’. This is certainly true in the UK. The recent BIS report on ‘Trade Union Membership 2011’ confirmed that the union wage premium in the private sector is 8% (an increase of 2.3% in past year). Although in the public sector the union wage premium is a massive 18%, it fell by 2% last year as austerity cuts and the pay freeze kicked in:
http://www.bis.gov.uk/assets/biscore/employment-matters/docs/t/12-p77-trade-union-membership-2011

In the run up to last year’s 30 November national pensions strike, the Guardian’s Seumas Milne made a similar point to Professor Gordon's but in respect of the UK - ‘a crucial factor in accelerating inequality over a generation has been the weakening of trade unions. Between 1975 and 2008 the share of wages and salaries in national income fell from 65% to 53%, while corporate profits swelled’:
http://www.guardian.co.uk/commentisfree/2011/nov/28/public-service-workers-strike