IBM isn't the company of the monopoly any more. They haven't been for a long time.
I've worked with/for IBM's service division, and the truth is, that group will work with whatever you put in front of it, as long as you pay your bill.
On the hardware side, IBM's PC servers traditionally ran Netware, NT or SCO Unix. SCO hasn't been a competitive UNIX system for at least six or eight years, but for the fact that a lot of prepackaged turnkey systems were developed for it in the 80s.
Linux comes along, supports SCO binaries (the aforementioned turnkey systems), is free, has a wide base of support (thousands of geeks with Linux exposure that wouldn't know SCO if it hit them over the head)... for the hardware guys, that's GOT to look like a win.
As a software company, the fact that Linux has almost everything that the big boys - Solaris, HPUX and AIX - have (far more than SCO), has open source code so that IBM can pretty easily develop what it doesn't, and the fact that it can be made to work with more-or-less everything IBM sells, from PC Servers to giant mainframes, well, that's got to look like a win, too. Even better, the lion's share of the development costs are done free of charge!
As a service company, we're talking about selling a single platform. IBM's sold single platforms for ages in its mid and high-end products. With Linux, it's single platform all the way from biggest Iron to the leastest workstation. That makes it easier to support, easier to migrate, and easier to scale. What's not to love?
In the end, I think IBM is sincere in adopting Linux. It's a sane thing for IBM to do. IBM has distinctly NOT embraced a single Linux, and it has also distinctly not branded its own. Since we're ~3 years into IBM's Linux marketing, I think it's safe to say that IBM has done the right things so far.