Hi Mike, thanks for your comments.
Yes, you can control everything if you want, (the main loop priority is just a value in a header). More: you can change priorities at runtime (but not in the m168, due it's small RAM).
About the footprint, this is de avrsize output for the mega168 (with the size optimizations flag):
Program: 5260 bytes (32.1% Full)
(.text + .data + .bootloader)
Data: 900 bytes (87.9% Full)
(.data + .bss + .noinit)
I know it lets small RAM to the user, but in a 328, a 1280 or a 644 it's much better.
Regards,
Julián