microSD and OLED Display
More progress these past few nights. I got the microSD card adapter and OLED display working on the breadboard. The Adafruit sample code for the display uses software SPI (bit-banging) while the SD card library uses the AVR's hardware SPI. The two methods won't play well together, so I'll have to rework the Adafruit example a bit so it uses the hardware.
The OLED display is awesome. It's 0.96" diagonal and the text is very crisp... even off angle. It's a graphics display, so you can also draw to it. I'll probably just use it for text, though. The full-blown sample application from Adafruit takes up about two thirds of the flash on an Arduino Pro Mini!
Everything on the board so far is running off a single LiPo battery through a 3.3V regulator. As I relearned, it's important to select a low-dropout (LDO) regulator. I grabbed one from a box of spare parts I had, only to discover that the regulator had a dropout voltage of around 1V. Since the initial voltage of the LiPo is around 4.18V, that means the regulator will shut off at around 4.3V (higher than my input voltage!). I replaced it with a LDO (Microchip MCP1700), which has a dropout voltage around 178 mV (at full 250 mA load). It's working nicely.
Next up... an SRAM chip to allow for larger data buffers.