But do walkouts of the type represented by yesterday's London Underground/Metronet strike or last week's audacious and highly disciplined stoppage by the Prison Officers' Association represent the future of trade unionism, or are they something of an aberration?

A discussion paper drawn up for next week's TUC annual conference reaffirms what most of us have noticed already – that the typical trade union member is increasingly likely to be a graduate woman professional in office clothes than a blue-collar man in overalls.

However, while the paper, drawn up by the TUC's head of organisation and services, Tom Wilson, predicts further gains among "associate professionals" – a huge group, running from nurses to journalists, estate agents to train drivers – the picture is far from entirely rosy.

Looking behind the headline figures, the TUC acknowledges, union density (that is, the proportion of employees who are members) among associate professionals fell from 48% in 1995 to 42% in 2005, and is predicted to fall further.

What has really happened is that the labour force itself has changed. Unions haven't become better at targeting white collar professionals – it is just that there are more of them out there.

The report, titled The Future of Unions, records falls in union density across the board between 1995 and 2005, from 22% to 19% among managers and senior officials, from 38% to 25% among skilled trades and occupations, and from 28% to 21% in elementary occupations.

Despite this, Wilson remains upbeat, pointing out that most unions are growing, and that they are "growing fast among the key group of working people who are the core of the knowledge economy".

Among the unions that put on members during 2006 were both the "traditional" manual unions – construction union Ucatt added 6.4% last year – and white-collar organisations such as the National Union of Teachers (up 6.1%) and Association of Teachers and Lecturers (up 4.1%).

If Wilson is right, even more trade union members in another five years are likely to university educated and working in the white collar services sector – simply because that is the future of employment as a whole.

But for as long as train operators need platform staff, maintenance crews and train drivers, union leaders like the RMT's Bob Crowe are still going to be around.

UPDATE: It's also worth noting, of course, that white collar workers and associate professionals may be just as likely to find themselves on a picket line. An important Unison committee rejected an improved local government pay offer yesterday, and if a ballot goes ahead, the union may be calling its members out as early as November.