In about 6 weeks, PJRC will start selling a shield meant for these types of projects. The PCBs went into manufacturing this week, so this product has finally moved from the lengthy planning & design phase to the beginning of production. Normally I resist the vaporware urge to mention stuff in development which has no clear delivery date. Since this is now in production, you can be pretty sure we'll be selling these before Chistmas.
This new shield will have 5V buffers (compatible with 24 Mbit SPI to long APA102 strips) and digital logic for proper SPI sharing, and 8 Mbyte fast SPI flash for storage of images & sound clips, and 10DOF fast motion sensing, and a 2 watt audio amp for props & costumes with a sound component. The form factor will be 1.9 by 0.7 inches, where the extra 1/4 inch length on each end allows mounting holes and 4 pins. The end overhanding the USB will have pads for power and the speaker, and the side overhanging Teensy's pushbutton will have 4 pads that mate 1:1 with APA102.
A tremendous amount of work has gone into this shield's design, to (hopefully) avoid some of the very thorny problems. For example, even though the PCB doesn't need complex routing, a 4 layer design is used to allow the rapidly changing LED current to flow through distributed planes, and around a small circular cut-out area, for minimal interference with the magnetometer. The amp uses differential input with connection to Teensy's AGND pin, to minimize ground loop issues, and filtering of ultrasonic harmonics from the DAC. Of course, it's designed for proper SPI sharing and fully compatible with the fast 24 Mbit/sec rate.
I'm working on examples for LED Poi and hula hoops (the motion activated hoop may require 2 boards...) and some sound reactive stuff too, using FastLED and SerialFlash libraries. Farther out than the initial release in 6 weeks, I'm going to work on JPEG and GIF (with animation) libraries and other supporting software that will be extremely useful in these applications. MTP will be added to Teensyduino and a SerialFlash example, so you'll be able to access the SPI flash as a normal folder and simply copy files to it (modern Windows and Linux natively support MTP... Macs require a 3rd party program, but MTP support does exist as an addon). My goal is to make these challenging POV and sound reactive motion sensing projects much, much easier for everyone.
The upcoming shield will sell in 2 flavors. The full version will be approx $25. A low cost version which omits the motion sensors will be $10.