Do the Oldham East & Saddleworth opinion polls mean it's all over?

"On the face of it, it looks as though Labour have it in the bag," says Anthony Wells on UK Polling Report. But he goes on to enter some caveats:

Firstly this poll itself has the potential to change the result. By demonstrating to supporters of the coalition that the Liberal Democrats the best placed to beat Labour, it could encourage Conservative tactical voting for the Liberal Democrats. Against that is that the Populus data suggests there is already a substantial amount of Conservative tactical voting for the Lib Dems, and there would need to be truly massive tactical voting to make up the large Labour lead.

Secondly, there were a substantial amount of former Conservatives and Liberal Democrats who said don’t know or refused to give a voting intention. Overall, 24% of 2010 Tory voters did not provide a voting intention to Populus, a third of 2010 Lib Dem voters did not. ICM found a very similar pattern. While these voters have the potential to make a difference, remember they are already factored into ICM and Populus’s topline figures, as they assume a proportion of them will vote for the party they did last time (in the case of Populus it reduced the Labour lead from 19 points to 17 points).

Finally I wanted to look a the record of by-election polls, which has the reputation of being somewhat patchy.

He goes on to examine that record in by-elections held during the last parliament.

Incidentally, Anthony also says:

We don’t have detailled tables from Survation, the third company to produce a poll for Oldham East and Saddleworth, so we can’t draw any firm conclusions about why their poll showed the Lib Dems doing much better.