The SGI purchase I can understand as Rackable wants to be a one stop shop for their customers. SGI has been on a downward spiral for years. Who needs to buy a super expensive custom OS/Hardware to do graphics processing or HPC when I can get commodity hardware and do just as well if not better?
The Sun purchase doesn't make much sense to me. IBM has no need/want for a new OS and it sure can't be for the sales force/contacts that Sun has. I can think two things that it "might" want. One is to expand their server hardware line to AMD though why? IBM is well known for their "better than Intel" line of Intel x86 chips and Sun is of the same quality generation in AMD terms. The other thing I can think of is that IBM wants to take full control of the Java Stack. They have a heavy investment in WebSphere and is one of their more profitable product lines so why not take control all the way down if you can?