A couple of historical points first: IBM's mainframe business is from a bygone era. IBM established mainframes and with OS/360[1] had a remarkable system. The custom was for hardware manufacturers to supply an O/S for their equipment at the same time as the equipment was being released. The last dying gasps of this era were the Data General Eagle[2] and the IBM PC[3].
With the hardware and system software coming from the same vendor, it was an industry standard to lock in customers as much as possible. This wasn't particularly sinister at the time, because competition was wide-open.
Hardware vendor lock in was doomed with the release of Unix Version 7 in 1979. Version 7 was the first portable O/S that didn't have its own "native" hardware. It was also the first Open system in the sense that every system call was fully documented and available for developers.
The original IBM PC attempted to lock in users with PC DOS (the other two promised OSen weren't released until many months after the first PCs were being shipped and so poorly done that they never received any significant market share), but failed when they simultaneously rebranded 3rd party system software (MS DOS) and used what later became commodity hardware because the original IBM PC had completely open hardware specs a la the Apple ][.
AT & T Unix as a proprietary system has been dying a slow death for the last couple decades. It will be dead and buried whenever the current SCO/Caldera lawsuits & bankruptcy is settled. However, it lives on in that interface it used was still completely open and its intellectual offspring include Linux, NetBSD, FreeBSD, Darwin, etc. There is much software from the 1980s that can still run on modern systems, so long as source code is available.
Unix killed off[4] nearly all of the proprietary main frame market for time-sharing. All of IBM's original competition Honeywell, GE, CDC, Burroughs, etc. are dead. This is the rough timeline of mainframe computing.
Moving forward to today, the modern mainframe is a highly parallel system. We have Beowulf Clusters, SGI Altix blades and even (gasp) Microsoft HPC 2008 running on Crays.
That means if IBM can convince the DOJ that the relevant computing market is one where it holds a minority share
I believe that to be exactly the case. I know from following the Linux Kernel Mailing List that IBM certainly contributes to mainframe support on its own machines and big iron support on Linux in general.
They are NOT the bad guys any more.
[1] Frederick Brooks, Mythical Man-Month.
[2] Tracy Kidder, The Soul of a New Machine.
[3] The IBM PC was in the unique position of having itself square in the middle of the passing of the era.
[4] With my help, I'm happy to say.
The real bad guy in these particular lock-ins to IBM mainframes is generally the millions of lines of proprietary legacy spaghetti code, some of it in long forgotten computer languages, that would have to be rewritten, and retested, if the user were to migrate to these cheaper options. That is, for many organizations, a daunting task.