Since 2008, Google decided to allow brand bidding through their adwords platform in the UK and Ireland, the reason for this was to bring it in line with their US operations which has allowed brand bidding since 2004.
Essentially, this meant that a brand could no longer hide behind their trademark for brand protection in the pay per click arena. Google was essentially allowing rival companies operating in the same sector to bid on each others brand.
The attraction for competitors bidding on one another was to try to lure traffic to their own site and look to convert these potential leads to sales and together with bidding on your own brand term and joined up SEO strategy, it provides an opportunity to seize a monopoly within the SERPS.
Since introduced to the UK & Ireland markets, take-up of this strategy on brands bidding on competitors has been limited, however it has been more active within the affiliate sector where specific programs use this as a strategic tool to drive sales.
An interesting post on Econsultancy can be found on the latest developments on brand bidding after a case went through to the EU and their response to this contested policy.