Thanks Mr. Vker!

Great find.

Unfortunately my flight arrives after they close.

For those who find this thread in the future: I called around and found that Reykjavik Excursions (http://www.re.is/) has luggage storage at the BSI (main bus terminal in Reykjavik) for 500 kr/day.

They are the same people who operate the Flybus (which runs directly to the BSI in Reykjavik after each arrival at KEF.)

The luggage storage hours are technically 0400-2300, but I guess someone is there all the time if you have a later arrival or departure.

Cheers!