After my misadventure with FTDIgate, I decided to try the CP2102. Unfortunately, I discovered that the breakout was not exactly pin compatible with the FT232RL.

(also upside down). The main issue is that the ground pin was not in the right spot. Not insurmountable for the purposes of as test, just use jumpers...

So I did, but.... there was still something not quite right. The board was recognized by the OS, but attempts to upload the usual blink sketch gives me:

Obviously something to do with the uploader, so on the internets I find:

http://www.kerrywong.com/blog/wp-content/uploads/2010/07/CP2102_ArduinoSch.png

Thanks, Kerry Wong, whoever you are. The difference lies in the Rx line where I currently have a 10K resistor between the Serial breakout and the ATmega, in accordance with the schematic I originally used:

So, shorting out the 10K resistor and trying again: voila.

Don't ask me why, I haven't dug into the details. Just glad when some things work out once in a while.