The documentation for keywords (on page http://clojure.org/reader) specifically states that '.' is not allowed as part of a keyword name; however '.' is specifically useful. For example, several web frameworks for Clojure use keywords to represent HTML elements, using CSS selector syntax (i.e., :div.important is equivalent to <div class='important'>).
In any case, the use of '.' is not checked by the reader and it is generally useful.
I would like to see '.' officially allowed (in the documentation). Further, I'd like to see additional details about which punctuation characters are allowed (my own web framework uses '&', '?' and '>' inside keywords for various purposes ... again, current reader implementation does not forbid this, but if a future reader will reject it, I'd like to know now).