Well, anyway, I've made a basic "Nixie Clock" skin for Flip Clock, using tweaked versions of ciroip's graphics. These are graphics files for version 0.2.1, other versions might have differently named graphics.

Untar the nixies.tar file into your /usr/share/flipclock folder. Make a backup of that folder first!

Notes:

• I just use the no-number version of the nixie tube (null.png) for all three frames of the flip animation. The problem is that there doesn't seem to be any transparency for the animation, so there's a weird flicker when the numbers change. I probably need to make a version of the no-number tube that has the background included. Sigh.
• If you are using the 12 hour clock, I use an awful nixie tube letter P in the tens column to punish you. This probably looks even weirder at around 10pm. Anyone got an actually good idea?
• If you want my source image, here it is: nixie-full.xcf I used Inkscape to break up the svg that ciroip gave us, then I used Gimp to rebuild it into a multi-layer picture. All the numbers are there, you just have to make their layer visible.

Screenshot: