Take a look at www.gravitech.usThey have a whole line of shields for Nano's.Maybe adapt them to use with Promini?
At the same time, a promini is not much more than a surface mount breakout board.I'd just as soon put the chip on a custom board with other needed components, get a set of PCBs from iteadstudio (or whomever) for $10 and then not have to deal with all the headers & stuff.
This idea is something I've thought about for a while. Mainly because seeedstudio's $10 PCBs are 1.9" square and the arduino shield needs to be 2.1" to span both rows of headers.
Don't quote me, but I'm pretty sure I've seen shields done on the 5 cm square boards. The header pin holes are awfully close to the edges, but they appear to fit.
If you have any links, I'd love to see how it came out.
maybe have drill holes that are broken open by the edge of the board as guides.
One day, I'll get someone to teach me how to solder properly - I definitely want to learn. Just always running out of time...