Any inroads that IBM has made into business would then belong to Apple and it would be a major push for OS X into business. IBM could then license singularly to Apple to run OS X on their boxes (SE) with Trusted Computing, M$ would then be in jeopardy as would DELL.
IBM already has a reputation for developing and building some great hardware, though equally pricey as Apple's so it would make more sense for this to happen than for Apple to allow any generic x86 box to run OS X.
Society in general wouldn't notice much... unless they paid attention to what computer their kids were using in schools. A trend would continue that pushed many to get into the OS early for their job years. It's been an old stereotype that Apple was great for grade schools, but in HS and College, they need Windows machines to prepare them for the working life in the real world. Well this could change the face of that notion in a large brushing motion.
M$ would now be running scared and forced to do something, like sue left and right, anything to prevent something like this from happening. I think actually, like the Intel switch, (M$) may have been behind AMD sueing Intel. I think IBM and Apple would be sued for this merger.
There are some pitfalls as with any merger, you have people on both sides that fill the same position. Well, one has to go, and this is what happens at the negotiating table. Middle management is totally destroyed in the process, upper management has parachutes, and the distribution channels will have to cope with the new scope of working together. The logistics of this alone would be dizzying.
This of course would never happen, not while Steve Jobs is the man. He'd sooner go private. His company is in the black ink and all is well, as long as his people continue to create, innovate, and produce.