Once bitter rivals, Scott McNealy and Steve Ballmer used a press conference to present a friendlier face, frequently bordering on saccharine, discussing a relationship that has started to bear its first fruit in the form of web services interoperability.

Such is the complex nature of the IT business, that in announcing interoperability between the Sun-backed Liberty Alliance standards and the Microsoft-IBM-backed WS-* specs, the two men managed to make it sound like they were aligning against IBM.

The bottom line is we have Solaris and Windows playing nice in a unique and quite unexpected way, right across the board, McNealy said. Sun and Microsoft now have very constructive competition, as opposed to just a free for all brawl, he said.

Two specs for allowing WS-Federation and Liberty’s ID-FF to interoperate by sharing security assertions between applications were published — the Web Single Sign-On Metadata Exchange Protocol (Web SSO MEX) and Web Single Sign-On Interoperability Profile (Web SSO Interop Profile).

The specs will eventually be submitted to a standards body for ratification, and both Sun and Microsoft have committed to use them in the Java Enterprise System and Windows Server. Sun plans to release a Java implementation of the specs to the community.

What we’re announcing today is all the protocol specifications to try to bring together the Liberty world and the WS-* environment, Ballmer said.

An executive from General Motors took the stage to explain how he had tied his Windows and Active Directory identity systems to a portal based on Sun software, using web single sign-on enabled by the new specs.

The strategy is to make Sun’s and Microsoft’ Java and .NET stacks attractive platforms for enterprise computing, by allowing them to interoperate, and to migrate companies away from IBM mainframes, the CEOs said.

It took longer than we expected, but the backbone of enterprise computing is moving off of mainframes, Ballmer said. Scott and I will disagree where they [customers] should go, we will disagree religiously, but we will agree that they should go.

When customers ask us ‘Shall I use .NET or Java enterprise system?’ we are going to disagree, McNealy added. Systems integrator partners will have to give the neutral advice on that count, he said.

It’s mankind versus IBM Global Services, and here I have three representatives of mankind, McNealy said, introducing executives from EDS, NEC and Accenture. We feel comfortable that this gang won’t be pushing an IBM mainframe.

We’ve got 200 billion lines of legacy code running on legacy mainframes… part of modernization is to take that apart, said EDS’s Charlie Feld. We’ve got to free up money to invest, which means getting people off very expensive legacy environments.

While the two CEOs made much of their jabs at IBM’s services and mainframes businesses, making Java and .NET stacks more interoperable doesn’t mean just Sun gets to sell its Java gear. IBM is also a Java shop.

Finding these integration points, where Sun and Microsoft can interoperate while remaining competitive, does not stop at aligning their web services protocols.

The two companies have inked a protocol license agreement around Windows Terminal Services, Ballmer said. That deal, coupled with Sun’s acquisition earlier this week of Tarantella Inc, will enable even more interoperability.

Sun Ray can access Windows applications through Windows Terminal Services, Ballmer said. Tarantella is a value-added product that facilitates that interoperability between Microsoft gear and Sun gear.

All this friendliness comes after a year’s work following the shocking April 2004 antitrust settlement between the two firms, which saw Sun walk away with $1.95bn and a ten-year technology cooperation commitment.

McNealy said that the first six months of the post-settlement work, as Microsoft and Sun engineers got to know each other and talk a common language, was tough going. Now, Ballmer finishes McNealy’s sentences, he said.

Times over the last year it seemed like centrifugal force or antibodies were going to make it not happen, he said. But that has changed, he said. Microsoft will be a major sponsor of Java One next month, who’d have thought it?