All of my boards are 3.3V and run at 12MHz. When I make the bootloader
I change F_CPU in the bootloader makefile. The delay function
works fine.
However, you could run into problems with some libraries. One customer
reported issues with NewSoftSerial. The issue was a timing
table that looks at F_CPU. There were constants for 8MHz and 16MHz
but none for 12MHz. I wrote a Perl script that added the constants
for 12MHz and the library works. The details are at --
http://wiblocks.luciani.org/docs/app-notes/software-serial.html
There could be other libraries with similar issues. These are usually easy
to fix.
(* jcl *)