I imagine that its not just IBM, all large corporations should be like that. Most off the machines that they have are usually just used for typic docs and running excel. When I worked at shell I got an evo (I think) compaq that was about4 or more years old. The thing sucked, but it was more than willing to do my spreadsheets and documents, although my ppt for my final presentation nearly killed it (my 20 minute presentation was forced into a 1.5 hours with lots of graphics).
That being said, from a corporate point of view, why do they need to go out and buy new machines when the old ones are working fine. I know that its nice to work on new computers, but sometimes thats just not going to happen. Even with your coding I doubt that you need a sophisticated machine, although I bet that a nice fat monitor would be a real plus.
Although I am curious as to why you didn't get a lenovo notebook, they are really nice and that employee discount should be real nice
PS, what part of the country do they have you in, I am currently seeking a position with them and want to know that I am not going to be the only person potentially catching flack for having a mac.