chairman and CEO Marc Benioff has said that this year may well be the last time the company attends Oracle's Openworld conference in San Francisco.

Benioff's comments come after Oracle cancelled his keynote speech at Openworld 2011, which he believed to be Oracle CEO Larry Ellison's reaction to negative Twitter comments he made on Ellison's opening keynote on Sunday.

"My guess is that this is probably our swan song," Benioff said.

Following the cancellation, Oracle offered Benioff another time slot on Thursday morning - when he is on his way to speak at another event in Ohio.

Using Twitter, he drummed up enough interest to hold a rogue keynote address to an overflowing room at a local San Francisco restaurant at the time he had been originally scheduled to speak.

"Larry just cancelled my keynote tomorrow!" Benioff Tweeted, inviting people to join him at the local restaurant instead. "Beware of the false cloud," he added. In another message on Twitter, Benioff said "The show must go on! Sorry Larry!"

Benioff also sent a message to Michael Dell, CEO of Dell, saying that he looked forward to seeing him and hearing him talk at Dell World. "OK, please don't cancel me like #oow11." The Dell event is scheduled for 12 October in Austin, Texas.

However in person, Benioff appeared to be more apologetic towards Ellison.

"[On Twitter] I said Larry had set a low bar in his keynote. I apologise to Larry for that comment, I'm sorry it upset him. I was just being competitive and he took it personally. He did train me. I am just trying to be a good student and do what he would have done in a similar situation. He does a good job of being competitive," said Benioff, quoting Oracle's recent press release that accused Autonomy's CEO Mike Lynch of being a liar.

"That [Autonomy accusation] was an aggressive move, and Larry's about aggressive moves. That's how he taught me in business. That's why I was very surprised and disappointed that he cancelled. In my world, it's just tennis, it's not personal, and I would never want it to interfere with our relationship," he added. paid $1 million to do a keynote speech at Openworld, which Benioff expects to be refunded.

"They told us yesterday they're going to give us our million dollars back, so we'll see if we get the cheque," he said.

Meanwhile, an Oracle spokesperson said: "Due to the overwhelming attendance at Oracle OpenWorld we had to make several session changes.

"The Executive Solution Session was moved to Thursday at 8:00am in the Novellus Theatre."

Benioff had re-tweeted a number of comments critical of Oracle and Ellison's keynote speech. He also holds that Salesforce is a real "cloud" company while Oracle is not.