Given that Apple's largest sales losses were in the larger business size categories, it looks like Apple's share in business (and maybe the market's overall concentration) is concentrated in smaller enterprises. This isn't particularly surprising historically, but I'd have been willing to believe that enterprises with high-dollar mobile workers with the clout to choose their own hardware might be more densely gathered in larger businesses than the share numbers suggest.
It looks like Apple sales of PCs to enterprise is likely trivial, that Apple could suffer >80% volume loss in the largest business segments without impacting share growth much. To the extent there's a market to chase there, it's wide open for Apple to take share.
But there are businesses Apple can bring into the fold without selling Macs. Apple recently ditched its WindowsMobile-based POS system in its retail stores, and has been deluged with requests by other businesses to deploy Apple's POS system. Even if Apple doesn't roll out its payment system as a commercial product (and find a way to profit as a high-volume payment processor), Apple can sell hardware to people looking to deploy third-party iPhone-based POS systems.
Eventually, if Apple continues to deliver high-value desktop and portable solutions, its sales will expand beyond the mobile devices currently on fire and begin to make inroads on the big-iron back-ends and other enterprise domains that perpetuate anti-Apple inertia. If this segment of the market is wide open, it's simply a question of timing and targeting.
Apple seems to surprise people with the markets its able to target, and I expect this trend to continue.