Al Qaeda control of the kingdom does not necessarily mean either the overthrow of the House of Saud -- at least at this stage -- or the more immediate destruction of the country's oil infrastructure or disruption of oil exports. The kingdom is infinitely more valuable with its oil sector intact. Al Qaeda will concentrate on weakening the regime and driving Westerners from the Arabian Peninsula for the foreseeable future.
Al Qaeda does not want to trigger a U.S. invasion or any other serious political backlash like a full-scale revolution or a fracturing of the country that would restrict Riyadh's political reach. If it can find a cooperative branch or a support base within the royal family, then the "regime" could persist -- at least in name -- even as Riyadh's political orientation shifts.
There could be short-term reasons for not completely displacing the House of Saud. The most immediate is money. Al Qaeda has long relied on financing from the kingdom. There are persistent rumors that some members of the royal family back the militants financially and politically. The U.S. Central Intelligence Agency reports that al Qaeda has spent around $30 million annually to finance operations. The U.S. commission looking into the events of Sept. 11 concluded that most of those funds came from "witting and unwitting donors, primarily in Persian Gulf countries, especially Saudi Arabia." Some of the money is believed to have passed through charities.
Plastic also has a recent post on Saudi Arabia full of links that might help put things in context.