Expect no miraculous joint announcement from the event, which will be a 75-minute joint interview conducted by tech columnists from conference organizers The Wall Street Journal.

Walt Mossberg, a co-producer of the conference who will interview the execs on-stage along with colleague Kara Swisher, said they simply invited Gates and Jobs to do the interview.

We expect to use the occasion to get them to reflect on both the past – how we got here – and the future, said Mossberg, who is personal tech columnist at the Journal, in an email. He declined to give any color about the questions he and Swisher are preparing, or any additional information.

Most likely, Gates and Jobs will use the occasion to do some friendly sparring on their polar-opposite philosophies on personal computing. Jobs may bang on about the benefits of a software-hardware approach, while Gates may rattle off the joys of partnering with hardware partners.

They also will probably try to raise each other’s hackles with new details about forthcoming rival products. Jobs, for sure, will talk up the forthcoming release of Apple’s iPhone, while Gates may respond with details of a Zune phone, which Microsoft has previously confirmed is on its roadmap but given no other details. Gates also is likely to point to a new version of Zune, Microsoft’s digital media player that competes with Apple’s more popular iPod.

Gates will no doubt spend time on his vision for the ‘digital home,’ as he has been fond of doing for some time now. And Jobs won’t miss an opportunity to tout Apple TV.

Audiences may even hear about Microsoft’s plans for the digital automobile. The company recently was awarded a patent for a car stereo that has a docking station for handheld digital devices, including MP3 players and handheld computers. Gates probably will also spend time on the latter, given Redmond’s push for ultra-mobile PCs, which are fully functioning PCs in a handheld design.

Whether either exec will have much to say about forthcoming enterprise technology is uncertain, especially given they have mostly stuck to consumer stuff at past D conferences. Jobs announced the AirPort Express Base Station at D in 2004, and previewed iTunes 4.9 at the event a year later. Gates last year gave some new details about Vista at D.

The pair have, on occasion, have had friendly spars in the past. And this is not the first time they have shared the stage. They appeared together at least once before, for a promotional gimmick in 1983. And in 1997, Gates appeared via satellite link during Jobs’ keynote at Macworld to announce a $150m investment in Apple and Mac versions of its software.

They have also dined together, offstage, at our conference in past years, Mossberg said. But as far as we know this is their first joint onstage interview.

The pair likely will appear as somewhat of an unintentional spoof of Apple’s TV campaign, which has a hip-looking slim guy representing a Mac and a tubby, dorky-looking man in a suit representing a PC.

Jobs, no doubt, will don his trademark black mockneck, blue jeans and trainers, while Gates will sport a slightly crumpled business shirt, khakis and disheveled helicopter hair.

Other speakers at this year’s D will include Google chief executive Eric Schmidt, Cisco chief John Chambers, Palm founder Jeff Hawkins, AOL founder Steve Case and Star Wars creator George Lucas.