madworm:
It seems to me that using a GPL-ed compiler + libraries implies that you must release the source code (or at least provide it upon request).
No, that's not correct:
Using the Arduino core and libraries for the firmware of a commercial product does not require you to release the source code for the firmware. The LGPL does, however, require you to make available object files that allow for the relinking of the firmware against updated versions of the Arduino core and libraries. Any modifications to the core and libraries must be released under the LGPL.
You mentioned that you "reproduced an Arduino on a breadboard". If your design is directly derived from the board schematics provided here on the Arduino site (a lot of copy and pasting from the Eagle .brd and .sch files) then technically you would be required to release your schematics. Granted, anyone trying to force you to release those schematics would probably just get laughed at by the community at large.