back when IBM was investigated for their mainframe monopoly, the mainframe basically _was_ the computer market (there was a smaller market in mini computers, but microcomputers/PCs were not a noticable factor)
today the entire mainframe market is happily ignored by almost all businesses using computers.
At this point, saying that IBM has a monopoly on the mainframe market and should be forced to separate their hardware from their software is very similar to saying that Apple has a monopoly on their market (they are the only ones who make the hardware, and they forbid anyone from using their software with anyone else's hardware)
It would be nice to have both open, but the law doesn't require it, and in the Apple vs Pystar lawsuit recently, Apple got the court to rule in their favor in a big way (namely that copyright forbade pystar from installing OSX on their computers, even though they had purchased the appropriate number of legal copies)
In many ways, apple's control of their market is much more significant to consumers than IBMs control of the mainframe market.