A 4-pin charlieplex on the Arduino. I ordered huge 10mm LEDs by mistake, and they don't really fit or show up on the camera well. Oh well, it works! Pics, schematics and code are in the blog post.
Just read the blog and it's the first article on charlieplexing I've read. I'm pretty thrilled at how it works, it's a very cool concept 
I do have a question - how many LED's could you have in a matrix before you see noticable flickering, and how many pins would that be using up?
I'm not sure. But it depends on how many are lit at the moment, assuming you're only giving time to those that are lit.
The number of LEDs in a charlieplex network of n pins in n * (n - 1). So this 4-pin has 12 LEDs, a 5 pin will have 20 and a 6 pin will have 30. If you had a really huge network, say a 10 pin network for 90 LEDs, things might get tricky. If they're all on, to flicker them at 60Hz, you'll have to run a timer at 5400Hz. Though, then you'll probably run into a dimming problem. Where each LED in a 12 LED matrix is lit about 1/12th of each second, the LEDs in the 90 LED matrix will only be lit 1/90th of each second.
This is also not appropriate for any POV toy either. Though it might be interesting to see what kinds of patterns that would make.