I would choose "a unit". I was taught that it needed a vowel sound. 'U''s short sound is vowel like, but a long 'U' at the beginning of a word soulds like a 'y', usually not considered a vowel.

This is why most people say 'an hour' and "a usual day".

The wikipedia article on 'Okay', uses the straightdope.com as one of the sources.

http://en.wikipedia.org/wiki/Okay

A 'couch potato' is a term for a lathargic person who is not associated with any form of aerobic activity.

Wikipedia has some good--though lengthy--information on both topics.

http://en.wikipedia.org/wiki/Backtick

http://en.wikipedia.org/wiki/Tilde