Sparteye, I would construe "semi-weekly" to mean at half-week intervals, or something.

As would I, and the writers of the American Heritage and Random House dictionaries. I offer "semiweekly" and "fortnightly" as complements, not synonyms.

I suspect that the confused use of the "bi" terms comes from people variously understanding the "bi" part to be "twice" (it happens twice in the designated period) or "two" (it happens every two of the designated period). The former are more on target with original meaning, but the latter are sufficiently numerous that the dual meanings have been established.

As to "biennial", I've always readily distinguished the meaning from "biannual", and I'm wishing I was near my etymological dictionaries right now, so I could chase down the history of the distinction between "annual" and "ennial", since my AHD is only telling me that "ennial" derives from L "annus", which gets us nowhere in the end.