Quite a few dictionaries list
uber- as a combining form or prefix. Hence, it can be attached to a word with a hyphen, as in the following, from the magazine
Backpacker in 2011:
During summer in the Smokies (p. 90), the uber-popular Cades Cove Road becomes a cyclist-only loop twice a week before 10 a.m.
Or, it can be joined seamlessly to a word, to form a new, un-hyphenated, word, as in this, from
Harper's Magazine, 2003:
Frank suggests we go to a west-side uberhip bar and restaurant called L2.
Of course, such usage doesn't qualify
uber to be accepted as a word in its own right.
However, there is now at least one dictionary,
Wiktionary, that lists it also as a separate word: an adjective meaning "Super; high-level; high-ranking" and an adverb meaning "Very; super". Wiktionary supplies a number of supporting quotes for both senses. (Click on their "quotations" linkl after each definition.)
Other examples are not hard to find. For example, the Lonely Planet guide to Turkey (2009): "Luckily, thanks largely to the uber cool cocktail bar, this place delivers."
So I'm satisfied
uber is now widely enough used to be accepted.