I grew tired of Googling each time I needed the correct button sequence to put iPods into their various modes, so I've put together this little table:

iPod Generation	Hard Reset	Diagnostic Mode	Disk Mode
1st Generation "Scroll Wheel"	Menu + Play	Previous + Center + Next	Previous + Next
2nd Generation "Touch Wheel"	Menu + Play	Previous + Center + Next	Previous + Next
3rd Generation "Touch Wheel"	Menu + Play	Previous + Center + Next	Previous + Next
4th Generation "Click Wheel"	Menu + Center	Previous + Center	Center + Play
5th Generation "Click Wheel"	Menu + Center	Previous + Center	Center + Play