Until today, I was running rEFIt 0.11 under Leopard 10.5.3 on an Intel MacBook without problems . After installing the 10.5.4 update and rebooting, neither rEFIt nor OS X would start. The "apple chime" sounded, a blank screen displayed, then it sounded like the CPU was racing because the fans turned on at high speed.
I attempted several reboots, then had to boot the install DVD and restore from TimeMachine. Afterward, I removed rEFIt, again installed 10.5.4, and now the system's booting fine.
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Can't duplicate on my Intel MacBook with 10.5.4