This year (2011) the brambles have bloomed earlier than normal - as have most flowering plants - after the warm dry spring. They normally flower in July and August but the bees are very happy to have had the benefit of extra food during the slim pickings of June.
Brambles provide both nectar and pollen; the latter is a pale grey-buff colour and when you see it in the hives it can appear as if the comb is going mouldy.
Right now the hedgerows in our area are full of bramble flowers flowing outwards on their out-stretched arching branches. In our new field (fingers crossed) one of the boundaries is a wall of bramble rising up about 10 feet. I think this is a indicator that the bees will be very at home here.