Depending on what IBM has planned for Sun's technologies, the mix of licenses could be a challenge, said Randall Colson, a partner at Haynes and Boone. For example, some industry analysts speculate that IBM wants to merge the best of Solaris into IBM's AIX Unix, which is offered under an IBM commercial license. If Sun has merged a third party's open-source code into Solaris, IBM may find barriers to merging Solaris with its proprietary AIX software.

Perhaps most complex for IBM would be the intricate that Sun entered into with Microsoft, which ended a long-standing lawsuit between them over Microsoft's alleged attempts to undermine Java.

The deal netted Sun almost $2 billion from Microsoft, including payments of $700 million for Sun to drop its Java lawsuit, and a further $900 million for a patent-sharing agreement that could be extended for as long as 10 years. IBM, whose software business depends heavily on Java, would need to pull those agreements apart to ensure nothing could interfere with its business or expose it to legal risk from Microsoft.

With reports of the due diligence work only a week old, it would be premature to assume that any talks under way have run into trouble, Bowen said. But the longer they take, the more uncertainty it creates for the customers and investors.

"It's fair to say that with every day that passes, it makes it seem a little less likely that this deal is going to happen," he said.