An analyst with Stratfor(1), one of the world's leading private intelligence providers, writes that today's attacks in London may have been geared to boost sagging confidence in al Qaeda followers. "The London bombings clearly give them renewed credibility in the Muslim and Arab world. They demonstrate that al Qaeda is still active and able to make its presence known in the major capitals of the world," said senior analyst Rodger Baker. Read more here(2).
There is also a report(3) that minutes before the explosions, British police informed the Israeli Embassy in London that they had received warnings of possible terror attacks in the city.

1. http://www.stratfor.com/index.php
2. http://www.stratfor.com/news/archive/050707-london-bombings.php
3. http://news.yahoo.com/news?tmpl=story&u=/ap/20050707/ap_on_re_mi_ea/israel_britain_explosions_1