It’s not actually stopped. Today was just a test. Admiral Allen suggested at a briefing earlier today that the cap will again be opened and oil collected with ships on the surface. New York Times:

“We’re encouraged by this development, but this isn’t over,” Thad W. Allen, the retired Coast Guard admiral who is overseeing the federal response to the spill, said in a statement on Thursday. “It remains likely that we will return to the containment process using this new stacking cap connected to the risers to attempt to collect up to 80,000 barrels of oil per day until the relief well is completed.”
Earlier on Thursday, Mr. Allen said that closing the well off using the containment cap would only be an interim measure, and that the company must still complete the relief wells it is working on in order to seal the well for good.