I left off the following components when I built the Boarduino: J1, C1, D1, C3, IC2. Could have left off some more but the main thing I wanted was to get rid of the 5V power supply and use the parts elsewhere.
The biggest surprise was that the 3.3v USB cable (TTL-232R 3.3V from Adafruit), actually puts 5V into pin 3 of JP7 (the header at the end where you program the Boarduino). I had assumed that 3.3v meant that all voltages were 3.3v. To get around this, I set the power select jumper to "ext" and used a 3.3v regulator (the same part used on the Adafruit XBee Adapter, Microchip P/N MCP1700-3302E/TO) to drop the voltage to 3.3v from a 4v sealed lead-acid battery (a PowerSonic PS-445 that I bought from BatteryWeb,com), which will be charged by a 4.5v, 100ma solar cell I bought from SparkFun (P/N PRT-07845).
It may sound like charging a 4 volt battery from a 4.5v solar cell will damage the battery, but with a fixed mounting it will only produce peak voltage for 1-2 hours per day and most of the time will be less. The battery is a bit of an overkill, but it will be running the Boarduino and an XBee-PRO ZNET 2.5 module that sucks 250mA when transmitting. I don't need that much distance, but my line of sight path isn't great, so I'm looking for more power to communicate. I also can't have the Boarduino sleep much of the time as I'm sampling electric fence voltages (animal containment, not frying humans) and I have to watch for a 1 millisecond, 10 kilovolt pulse that happens about once a second. I'm still writing software and hope to be able to make some time predictions for the next pulse so that I can use sleep more, but I figured that I should design the first unit to be able to run without sleeping the Boarduino.
Hope it helps!