Problems with AIX
The Register writes to the the delay of AIX 5.4:
Most notably, IBM has shifted from an out of order design with Power5's integer units to an in order design with Power6. Such a drastic change will require first AIX 5.4 and then a full software recompile should customers want to see maximum performance. IBM has yet to say anything at all about the efforts underway to deal with this issue.
This would be really interesting for ISV. It seems, you need a pre-Power6 version and a Power6-Version of your code to get maximum performance. I assume, this will lead to nice benchmarketing tricks with performance levels unreachable for users with standard software from ISV.