What I had been looking for was a way to easily deal with multiple nixie tube digits in various projects that I was working on. I really wanted an easy way to connect many digits together with minimum digit spacing, and have the digits controlled by a simple serial interface. You'll find all the schematics and board layouts are available through the links in this instructable, released under a creative commons license.
Voila, the nixie tube board that I came up with supports two IN-12A type nixie tubes via two printed circuit mounting phenolic sockets. The nixie tube board is designed to be supported by a nixie driver board below it, which permits a microcontroller ( Arduino, etc.) to address the two nixie tube digits, and via a shift register chain, multiple pairs of nixie tube digits. The edge connecting header pins easily allow the multiple digits to be physically connected and may be powered by an external high voltage supply. This densely packed configuration permits minimum digit spacing while threading power and serial data connections to all elements.