I haven't understood yet if Atmel µC usually required an external oscillator/resonator.
No, the ATmega168 includes in internal 8MHz rc-oscillator (which is /8 for 1MHz initially).
The Lilypad firmware will run correctly on boards with the internal oscillator, for NG/Diecimilla compatibility you need a 16MHz clock of some kind.
An additional caveat, once you've fused a chip for some sort of external clock, you'll need to either use an external clock during programming or use parallel programming to rescue the IC.
Atmels long shunning of internal resonators means most ATmega designs use one, even if they could have used the internal RC clock.