First, I do not see ARM as a X86 killer. Those processors are just to powerful for ARM to catch up to anytime soon. Simple tasks like web browsing can be really slow compared to any of the X86, including my Atom 330. Even with the power backing the Panda ES this is true.
Where I think ARM, is and will continue to excel, is building smart devices and or appliances. I see a day in the near future where A Rep Rap or Makerbot will come with it's own rolled in design and control, standalone. Smarter Routers that will combat the ever growing attacks on people's networks, as they try to steal your personal info for their gain. Robotics seem to already be gaining higher resource control solutions than were available just a few years ago.
What it is going to take is some differing SoC's that have different balances of their core units. For that Makerbot you will need balanced power from the CPU and GPU. For that Smart router you will not need much GPU but a good CPU. And for that Robot again you will not need much GPU but a good CPU and better "real time" processing.
Some of these will need multiple "boards" to carry out their roles. X86 has proven too expensive too really do any of these well. It also is wasteful of power in most cases.
So back on topic.
There are no RasPi killers out there that I see so far. The cubieboard is inexpensive but at $45 to $49 is 40% more expensive than a RasPi. It does have some extra features, but it does not have as powerful GPU. On the other hand it does have a bit more powerful Processor. Also I have seen no real goal stated for the cubieboard. I see it might be better at being a server but not quite up to snuff as a media center.
Anyone who decides to make a RasPi killer is going to have some hard choices to make, if they are trying to make profit. Conversely trying to build a device that will take RasPi market share, by having more features, yet keep the device inexpensive is in for some work also. All those features on a Panda board come at a price, about 5 times the price overall.
There are some other considerations as well, other than just price point. There is about twice the operating systems available for a Raspberry Pi as any other board currently offered. So wider exposure. And there are an awful lot of people involved in the RasPi, people that could not afford the other boards that are out there.
The Raspberry Pi has proven there is a market for inexpensive computing devices. The real question is will they be willing to put the time in to having developed software to make their board successful? An ARM board without software is a paper weight to most of us!
A Raspberry Pi in the hand is worth a lot more than any other board still out in the bush!