That's strange. I upgrade my First Gen Macbook from Tiger to Leopard and then on to Leopard Server with no problems.

Did you do an upgrade or a clean install? I did a clean install and then a restore from backup/migrate to return my files and settings each time.

Yesterday's upgrade of the iMac to Snow Leopard went smoothly with the only issue being that Quicksilver refused to load. A quick deletion of the plug-in directory (I think) solved it. I'll let you know more as it happens.