Part of the problem is that in the North of England, beer was traditionally served with a deep head in oversized glasses. In the South it was served "flat" in brim measure glasses (hold a pint when full to the brim). In recent years breweries have been promoting the use of sparklers - the gadgets which froth up the beer - in the South as well. To be fair, they did use lined glasses (oversized with a line marking a pint measure), but Southern drinkers were used to their glasses being full so these didn't find favour.

CAMRA regards this as a scam and continues to campaign for full pints.

Oh, and just to confuse matters, a court case decided that the head on Guinness, at least, is part of the pint - see http://newsgroups.derkeiler.com/Archive/Uk/uk.legal/2007-06/msg04258.html