In reply to limey:
Where you live has a lot to do with it. When I lived in Preston we used to go cragging throughout the winter, just exercising a bit of common-sense about which venues we went to and accepting that it would be a bit chilly. Sometimes this did make bouldering the best bet. They key was that lots of south-facing rock does stay clean throughout the year in the Peak.
Now that I live in Lochaber I find that the last rock-climbs of the year are normally in October, and the first of next year in March, or possibly even April. The issue is not so much the temperature, but the wet climate and the rareness of crags drying out properly. Oh yes, and having a snow-and-ice climbing season to keep us occupied!
However, if I lived in Gairloch...