Here's some interesting test results recently uploaded to OpenBenchmarking.org that compares the performance of ARM Cortex A8 and Cortex A9 cores running at 1.0GHz against an Intel Atom N450. All three systems running at 1.0GHz were also running Gentoo Linux. Clock-for-clock, can the latest-generation ARM Cortex-A9 take out the Intel Atom? For the most part, yes.
An independent user with an IGEP0020, ORIGEN, and a Samsung netbook had some fun with the Phoronix Test Suite and then shared the auto-generated data on OpenBenchmarking.org. The IGEP0020 is powered by a single-core ARM Cortex-A8 at 1.0 GHz while the ORIGEN has a dual-core Cortex-A9 that's clocked at 1.2GHz but was lowered to 1.0GHz during testing. The Samsung x86 netbook meanwhile has a dual-core (via Hyper-Threading) Intel Atom N450 that's clocked to 1.67GHz by default but was lowered to 1.0GHz for testing.
Making things more interested is that the Gentoo x86 benchmarks were done when using Gentoo x32 and x86_64 binaries. On the ARM side, Gentoo hardfp-built binaries were used. GCC 4.7.1 was the compiler for Gentoo armv7 and x86 but the compiler releases targeting the different hardware platforms were slightly different. Unfortunately this independent Linux benchmark contributor using the Phoronix Test Suite didn't provide any power consumption data with his results, which can be auto-monitored by our open-source benchmarking framework.
On the following pages are these Cortex-A8, Cortex-A9, and Intel Atom results. For some other interesting Phoronix ARM testing, see Ubuntu 12.10 Continues Strong On The PandaBoard ES, ARM Wrestling: Fedora 17 vs. Ubuntu Linux, Quad-Core ODROID-X Battles NVIDIA Tegra 3, Building A 96-Core Ubuntu ARM Solar-Powered Cluster, and a 12-Core ARM Cluster Benchmarked Against Atom, Ivy Bridge, Fusion.