westfw:
I don't think that it allows crystal-less USB operation, though.
deep breath.... argh you're right. i remember looking at the 64-pin F070 and then selecting the 64-pin 072 instead ... then forgot that the 070s don't operate crystal-less argh.
Sure fits nicely, though (SSOP20 vs SOL16, eh? Interesting the way that works out.)
yeh it does... but ngggggh the extra cost of the crystal takes the BOM up to around $0.70... is it worth it? i'm tempted to say yes.
flip/pinswap the reset switch so the reset signal doesn't have to snake around so much?
Or is that intentional?
the 5th pin on the switch means i'd have to move the ... no it doesn't: managed to shoe-horn it in. will update.
I'm a little worried about the uSD card having room to slide out...
i think it'll be ok. those components aren't that high (R8), LED (marked PWR). if nothing else the uSD card will go over the top of them. if there was a 1206 capacitor or a pinheader i'd be concerned.
Newer Arduinos have simplified the op-amp part to only use a single op-amp, which would theoretically permit use of a smaller package. (OTOH, they're using the send opamp as a buffer for the L led.)
... because it's there, most likely. any schematics around that you know of that i can copy? is it worth shaving the cost by reducing down to a single op-amp? (answer's yes...) so have to find a single-channel op-amp... what's equivalent? LMV321? http://www.ti.com/lit/ds/symlink/lmv358-n-q1.pdf says so...
although it doesn't look very much lower in cost!! LM321 data sheet, product information and support | TI.com $0.19 in 1k volumes...
UA741 UA741 data sheet, product information and support | TI.com is $0.12 that's better....
honestly the last time i used op-amps was at school, and that was... um... 1985? 
help!