Apple just patented the design of the MacBook Air, which is really the image we imagine when we talk about an Ultrabook.
Patent infringements on ornamental designs are not pursued as often as technology patents, and it is unlikely that Apple would go after Intel as long as the chipmaker is delivering the hardware for its computers, but this patent surely is interesting in the light of Intel's $300 million fund to support the launch of its Ultrabook idea. Could go Apple go after Acer, Asus and Samsung, once those firms are releasing Ultrabooks? Possibly.
However, history has told us that PCs look different than Apple computers and there may not be really an issue as Ultrabooks are unlikely to resemble the visual appeal of the MacBook Air.