If anyone here has any experience with the Leaflab Maple boards, you will know what a massive undertaking the porting of much of the AVR-based Arduino code over to the ARM architecture is. The Leaflab boards are great little Cortex-3 prototyping boards, with an Arduino derived IDE and a "wirish" port of the basic wiring stuff, but much of the promise of the hardware (DMA, advanced interrupt handling etc.) even to this day has yet to be really fully realised because of the at times painfully slow rate of porting across (or creation from scratch) of many of the libs we take for granted on the Arduino.
I have developed using the Maple boards, and to be honest, despite the raw hardware advantages, the Mega2560 is in many ways far more capable prototyping platform, apart from some niche applications. As a sage once said, "it's the software, stupid."
So, recent history ponts to where the challenge is. Building a good ARM dev board -- certainly not trivial, but pales into comparison to the effort required in building a complete software development environment that would be comparable to the Arduino ecosystem.
Having said all this, I might add I'm not surprised that the announced "team Arduino" development effort for an ARM based Arduino product is today a year old -- and not a public beta release in sight.
So good luck to Paul. But he's going to need more than good luck -- he's also going to need considerable community support if this platform is really going to take off.