I don't think ARM have too much to worry about yet.
The Tilera chips seem to require a lot more power than ARM cores. Fine for servers, which will hurt AMD and Intel's business, but ARM's core (sorry!) business strength is in mobile and embedded markets, not servers. Mobile and embedded customers consider power / watt pretty bloody important.
Even when you add up all the CPUs in all the data centres in the world, it's still peanuts compared to the *billions* of mobile and embedded chips out there. That's why Tilera's pricing starts at $many and rises very quickly to $lots.
ARM do need to get their 64-bit architecture out, but I don't think they're too worried about the server market; compared to mobile and embedded, it's unlikely to become a primary source of revenue for them.
Remember, too, that ARM's philosophy is that their cores can be coupled easily to other _specialist_ processors, such as GPUs and DSPs. Tilera's approach is to assume everyone wants the exact same core lots and lots of times. Both have their advantages and disadvantages, so ARM aren't necessarily going to lose out here. If most of your processing is better done on a DSP or GPU, you don't need lots of 64-bit cores as well: a bunch of 32-bit cores should be plenty to parcel out the jobs to the specialists.