Still interesting to think about. I'll leave it to other analysts to discuss potential overlap in the areas of hardware, operating systems, and databases. Instead I'll focus on the platforms of greatest interest to content technology people.
First, there's the future of Sun's meandering enterprise portal strategy. Early this decade, Sun joined other application server vendors by layering a portal product on top. Sun Portal Server was reasonably popular in academia, but otherwise never really took off, despite more recent efforts to spiff it up and put it into open source. Then Sun announced an intention to come out with its own version of open source portal platform Liferay (which we review in our portals research). IBM has been amenable to open source, but any commercialized Liferay platform would have to compete with the behemoth IBM WebSphere Portal Server. That's a tough one.
Sun has also been rumored over the years to have come close to acquiring a Web CMS or Collaboration vendor. Never happened. Sun has been working on various Social Software modules designed to boost its MySQL franchise, but they haven't been productized. Seems to me that IBM has a better history of bringing skunkworks projects to market, so if the deal goes down, maybe we'll see more Sun's R&D in this area.
Finally, there's the question of Java specs and standards. Sun has been steadily easing its hold over Java; hard to tell what an IBM acquisition would mean here.
I think the most important thing about any acquisition is this: Big Blue is no Borg. At least not on the software side. IBM is a highly diffuse and actually somewhat disorganized vendor that tends to acquire products, then routinely lose track of them. For current Sun licensees, maybe that's not a bad thing.