florinc,
Very nice! I like that you did it with 74HC595's instead of the MAX.
Just a thought regarding the memory issue, if you haven't considered it. For $2 and a few wires you can add an I2C 256K EEPROM to store your fonts, messages, reminders, tengu faces etc. I found that it's quite fast enough to pull this stuff out of EEPROM realtime.
I was also looking into adding a mic driven VU meter / tengu - sensitive to 3 frequency bands. (Did you bug my basement?) I was experimenting with Sparkfun's LM4970 breakout (http://www.sparkfun.com/commerce/product_info.php?products_id=8247) with inverters and filters to get the PWM that represented the 3 frequency bands back to the Arduino via an I2C ADC. I got very close, but all that analog stuff was frustrating.
I wonder if anyone knows of a nice chip that splits audio into 3 bands and outputs them as analog voltages?
Anyway, you now have a nice I/O platform (viewable at a distance) that you can do all sorts of things with. Enjoy 
FWIMW - http://farm4.static.flickr.com/3114/2628485492_85b7963e37.jpg and http://farm4.static.flickr.com/3103/2628485440_ca0a7db0fd.jpg
