Quoted from a reddit thread.
This could be big. Many a sysadmin might have sleepless nights soon enough.
EDIT: Since news and clarification arrived, I'll add it here.
Official website with details:
https://meltdownattack.com
TL;DR
There are two attacks exploiting similar ideas, called Meltdown and Spectre.
Meltdown affects all Intel CPU's going back a decade, and some select ARM CPU's. It is the more pressing issue of the two, and potentially compromises systems completely due to its power. Patches already went out on both Linux and Windows to mitigate it. Performance hit depends on workload, gaming not noticeably affected.
Spectre affects all CPU's aside from specialized microcontrollers and other low powered devices. It is harder to exploit but also harder to fix. The full consequences and effects of it are still unknown, but all major tech companies are taking steps to research and mitigate it.
Intel Press Release:
https://newsroom.intel.com/news/intel-responds-to-security-research-findings/
AMD Press Release:
https://www.amd.com/en/corporate/speculative-execution
Apple Press Release:
https://support.apple.com/en-us/HT208394
ARM Press Release:
https://developer.arm.com/support/security-update
Updated title of the thread to include other CPU companies.
esquared
Anandtech Forum Director