IBM says we cannot expand our physical memory on our 570's without upgrading all the existing DIMMS since all slots are currently full. Is it really so difficult for IBM to build servers that leave a couple vacant slots for future expansion? A technical explanation would be greatly appreciated. Thanks All.
is it really so difficult for customers to think about future growth ?! 
Now seriously - bigger memory modules means as well slower memory. Most clients want as fast memory as they can get for their needs. IBM gives you the option to purchase your frames with spare memory for future activation and assumes that you want the fastest memory for your estimated maximum memory needs.
Limitations are per node module (you have up to 4) + per frame
4 GB to 48 GB of 667 MHz DDR2 + 192 GB of 667 MHz DDR2 total or
16 GB to 96 GB of 533 MHz DDR2 + 384 GB of 533 MHz DDR2 total
32 GB to 192 GB of 400 MHz DDR2 + 768 GB of 400 MHz DDR2
there are additional requirements how many node modules need to have the same configuration - as far as I recall, you need to keep them the same or same or one level bigger (which slows down ALL memory)
Hope that helps
regards
zxmaus
Do you have cfr/rtf (econfig) configuration of your server? If no, ask IBM about it.
Over there I will get exact order information (current server Setup).
Which dimms do you have ?
You do not have to replace all memory only few dimms is enough. (depeding of your needs).
But in reality if you will not demand your self that you want free slots person what is doing configuration will try to use the smallest size of memory dimms in configurator.