I was sort of inspired by AdaFruit's FLORA. I put an ATTiny84 on a board with 8 LEDs and a very tiny tactile switch on a 1" diameter round board. On the back, I put a 12 mm coin battery holder and the ISP. The LEDs are arranged evenly around the outside. I designed in a series resistor for each LED, but in the prototype I stuffed in "0 ohm" resistors, and that seems to have worked out nicely.
I tried two different 0805 LEDs. I bought a set of super bright white ones, and a set of sort of ordinary green ones. I was surprised how astonishingly bright the white ones turned out to be. I asked the girls which they liked better and they picked the green ones. Since OSH Park boards come in sets of 3, they were the tie breaker for the second of the two earrings. The third one (with the bright white LEDs) will wind up being a pendant.
The firmware uses the button to change the blink pattern to one of four compiled in (with a short push), and to put the board to sleep (with a long push) and wake it up.
I was concerned that a CR1220 wouldn't have enough oomph to drive the whole thing, but so far it seems to be working just fine. No word yet on how long a single battery will last. If it makes it through a... wearing event... then that's at least met the goal.
Anyway, I've waited until now to share the design because I wasn't sure it was going to work, what with the concerns about the battery being able to supply enough current. But now that I've seen it go... The firmware is up on GitHub, and the board design is at OSH Park.