"Tories and Labour neck and neck in marginal seats" scream the headlines, and it looks like a setback for Tory hopes. There's an excellent blog by Anthony Wells called UK Polling Report which does a daily take on what the polls mean, with intelligent comment on their methodology and findings.
Here's his take on it. The Populus poll sampled Labour seats targetted by the Tories, excluding the closest 50 seats. So it polled Conservative targets 50-149, and sure enough, it found 38% Conservative, 38% Labour. At the 2005 election the share in those seats was Con 31.4%, Lab 45.3%. This represents a swing to the Tories of about 6.7% since then. As Mr Wells puts it:
… the swing this poll suggests in the marginals is the equivalent of a 10 point lead nationally, a larger lead than most polls from other companies have been showing in recent weeks.
This is hardly the Tory setback or the Labour boost that the headlines implied. It really does help to have a voice of calm reason explaining just what can be gleaned from the data.