According to this site: http://playthesystem.co.uk/hotspots.aspx
Barclaycard have not made this public yet, but as of July 2007 they are going to be offering 5% cashback on purchases for 6 months to existing customers only, on their existing credit card accounts. This beats other cashback credit card offers hands down. So if you don't have a barclaycard now is the time to get one here, ahead of the offer. If you have a Barclaycard and want the cashback deal, call to speak to Customer Loyalty and ask for it - there's a high probability it won't be rolled out automatically to all holders.
Barclaycard are going to be offering 5% cashback for 6 months from July on all purchases to existing customers. As it doesnt start till July there is still time to get your card from the Barclaycard site in order to qualify for this offer.
Existing members on the MSE forums have called Barclaycard and they have confirmed that this offer is genuine and will start in July.