ARM has several different processor families in production at present. The newest releases are the Cortex processors, which come in three series-- A, R, and M. (See what they did there?) The M processors are the weakest and cheapest, below $10 even in low quantities. The R processors, intended for real-time applications like anti-lock brakes in cars, are the next step up. The top of the heap are the Cortex A series. So far, the A8 and A9 have been released, and the A5 is scheduled for release in 2011. The A8 is the processor at the heart of some recent smartphones, like the Iphone 3GS, the Nexus One and the Droid, for example.
Of the A8 and A9, the A8 is the cheaper, slower one, running in the 500 MHz to 1 GHz range; the A9 can have multiple cores and run up to 2 GHz. To me, the A8 crosses the threshold where we can build embedded systems that connect to the internet, have decent performance without requiring tuning to make applications run fast, and have a price in the $100-200 range. There are certainly many processors that can handle internet traffic in that price zone, like every decent consumer-grade Ethernet switch built in the last 10 years, but those are machines optimized for niche tasks. What's new is that we're finally getting enough clearance above the minimum requirements that cheap, general-purpose systems can function on the internet.
So if you want to make an embedded device that uses a Cortex A8, what chips can you buy? ARM is an unusual company in that they don't produce chips themselves; they license their designs to manufacturers in return for royalties on each chip sold. Right now, ARM lists 7 public licensees of the A8 design; in addition to those listed, Alchip and Ziilabs are claiming to have licensed the design. Several of the licensees, such as Ziilabs and Broadcom, are targeting niche multimedia applications and will likely only sell to large manufacturers of stuff like DVD players and video cameras. Of the remaining companies, two have released general-purpose A8 processors: Texas Instruments and Freescale.
Freescale has released the i.MX5x series, with two subfamilies: i.MX50 and i.MX51. They cost $30-40 in low quantities but the sole distributor listed (Avnet) reports a lead time of 26 weeks for all parts.
TI has two series of A8 processors: OMAP3 and the not-quite-released-yet Sitara AM35xx series. The OMAP3 series has been around since 2008, and there are several embedded Linux boards (Gumstix Overo, Beagleboard based around the OMAP35xx series, though none have a Ethernet port in their stock configuration. The first Sitara, probably the AM3517, will likely be the cheapest of the lot, but assuming we want to limit ourselves to chips that we can actually order, that leaves us with four choices (before we consider packaging): OMAP3503, OMAP3515, OMAP3525, and OMAP3530.
The two higher-end OMAP35xx chips, the OMAP3525 and OMAP3530, include a TMS320 DSP onboard, which is not worth the cost in a general purpose tool. This leaves us with the OMAP3503 or OMAP3515. The major difference between the two is the PowerVR SGX graphics accelerator in the OMAP3515, which, like the DSP, isn't worth the cost. Until the AM3517 or AM3505 hit the distributors, I think the OMAP3503 is the winner. We'd have to choose between the three different packages the chip comes in, but Digikey only has the CBB package (a 515-pin ball-grid array, distinguished from the CBC package by the pin spacing of 0.40 mm rather than 0.50 mm).
In the words of Captain Stillman, "Load and fire the weapon, soldier!"