[chryuop]

No, katakana is also used for non foreigner words. It can be used for onomatopeic (spell) sounds, stressing a word, a word used with a meaning which is not the one described in dictionaries...
As per names I really don't know the rule. When I write to my friends in Japan I use kanji and both the kana to write their names. Above all at the beginning of the letter I use the name written in katakana. They have never corrected me...so either they are afraid of offending me or it is fine LOL (but I am not Japanese nor a very good speaker of the language, so you'd better listen to better sources hee hee).