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.