kawaii ne~~~e! i have been watching a ton of japanese shows, where they would use kawaii alot, and most of the time i see it in katakana, its almost always in katakana, i dont think once i have seen it in kanji (unless i missed it obviously), but a few times i have seen it in hiragana.
Emphasis Katakana has 2 purposes. One of course is for foreign origin words and the other is for special emphasis. I think I should add this little trivia fact to the lessons because it is used so much!
New students are often taught that katakana is for loan words, but there are actually a lot of examples like ケイタイ（携帯）where a Japanese word is commonly written with it. Somebody posted recently that animals with obscure kanji are often written in katakana. When I view those websites that list all the prefectural birds, trees, symbols, etc., I find most of the animals are written in katakana.
One day for fun, I was quizzing a Japanese friend about his hometown trivia. So I say: "OK, tell me your prefectural fish!"
He replies: カレイ which means flatfish
So to be a smart-ass I write: 鰈 (the kanji for flatfish) and he's like....."Yeah, Yeah....but we just write it カレイ"
--------------- Here's a couple other very common examples I can think of off hand:
ゴミgarbage (instead of 塵) タコ焼きtakoyaki (instead of 蛸焼き)
Then, of course, there is the "special emphasis" katakana usage, as mentioned in the prior post.