The Ultramobile seems to be in the mouth of the press these days but their definitions of it tends to vary. The original is called the UMPC (Ultramobile PC) and seems like an odd collection of PDAs and tablets with keyboards. Nowadays some extend the definition towards tablets. Some considers hybrids (aka Transformers and Yogas) to be ultramobiles.
My definition of an ultramobile is simple. Ultrabook form factor aka ultrathin notebook factor + mobile processor (10w and under power draw). Other characteristics are solid state storage and fanless. Are hybrids and convertibles like the Asus Transformers, ultramobiles? I personally prefer to put them into another category, hybrids, since they are true tablets as well once you remove the dock. ARM tried to push the concept before of the Smartbook (Smartphone + Notebook), which ARM powered notebooks are running some mobile OS, namely Android. That concept seems to have died out in favor of simply tablets and to a lesser extent hybrids. The Ultramobile distinguishes itself from Netbooks and Ultrabooks because of the processor power draw and fanless operation.
In a sense the first true successful Ultramobile might be the Samsung X303e Chromebook. I believe Samsung examined it to themselves that their Chromebook won't be as successful if it had followed the Netbook and Ultrabook template, which their previous Chromebooks did. Thus, they are pushing into this category. Asus and Lenovo seeing the signs of this are going into this as well. HP on the other hand, puts a foot in one and a foot in another, in both ultramobile (the HP 11) and the ultrabook (HP 14) category. If any thing for the future, the next Acer design I suspect may also be ultramobile (a new model), along with an ultrabook one (new model), but still keeping the C720 (netbook form factor).
I believe that for a new OS to truly break ground in the market, it must not compete in a form factor that is already dominated by an earlier OS (Android and iOS in smartphones and tablets for example, Windows in notebooks). The case does show when another OS tries to enter the form domain already dominated by another OS (Android in hybrids and all in ones vs. Windows, Windows vs. Android/iOS tablets), the intruder will have trouble fighting the entrenched incumbent. Thus the new OS contender has to encapsulate, become integral and then associate itself with a new hardware form factor, and that the ultimate break for ChromeOS has to come from these new breed of Ultramobile notebooks, not tablets in one side and netbooks/ultrabooks on the other.