Why on earth did nokia go with the minSD?? I can't speak for the rest of the world, but here (USA) miniSD cards are hard to come by in a brick and mortar store. MicroSD cards on the other hand ... those are relatively popular for cell phones. I have a feeling they will stop making miniSD cards before I stop using my N810.

Seriously ... can anyone shine some light on why Nokia chose the miniSD?

That I do not know but there are microSD cards that come with a miniSD converter. Just Googling microsd minisd gave me some links. I guess transflash minisd adapter would gave me even more. Also I already see 8GB mini SDHC cards already for quite good prices.

Because minis tend to have a better dollar-per-gig ratio and a simple plastic adapter allows any micro card to be used in a mini slot?

Actually it was because they decided to make it really small an they did cut back on funcionality. Here in Europe MiniSDs are also more expensive, harder to find and non standard (at least not as standard SDs).
I was waiting for the n810 and I had the money ready, but then I heard it was gonna use MiniSDs. It's not about the money, it's just that for me SD cards (and the price they cost, amount of memory available per Euro and compatibility with the cards I already have) are much more important that keyboard, GPS, transreflective screen and small size.
So yes, now Nokia has a smaller Internet Tablet. Great. I heard the next model is goint to be even smaller, as small as a matchbox. It won't have wifi, screen, speakers, bluetooth, memory, keys and a powerplug, though... It's the price for miniaturization...
I will keep using my n800.