Infosec lesson: if you're really trying to keep a project under wraps, don't take it to the Maker Faire and tell everyone about it.
I stopped by the Atmel booth (yes, Atmel had a booth there, as did a few other established tech companies) and chatted with Paul Rako, who now has a blog post up identifying my Long Skinny Mystery Item: it's a Geiger counter!
Specifically, it's a USB Geiger counter, shoehorned into a form factor that allows it to fit tidily behind a specific Russian tube, the СИ29БГ or SI-29BG (which any interested party could figure out in a moment from the photos, so there's no harm in my mentioning it now).
The board respin should be turning up in my mailbox momentarily; while OSH Park is a dirt-cheap (and good-quality) fab service for this sort of dinky board, the 2-week turn time for 2-layer boards is not conducive to a rapid development cycle.
I'll be carrying the assembled specimen (and maybe a bare board of the respin) around later today at the Design2Part show, just in case of picking up any better ideas for packaging the gadget than my .375 Weatherby notion. Also, there's a lot of info to be gathered this afternoon for completely unrelated projects both current and upcoming.
As far as turning it into an actual product... I still need to do at least some sort of market study, and determine what the supply of that particular Geiger tube looks like. No point launching a product and then discovering that there were only 237 pieces of the key component available worldwide, nor in cornering the market on that component and then finding that only 14 people actually want to buy the product.