I have made a list of firmwares that are or aren't compatible for each iDevice given their generation model.

Ipod Touch
1.1-3.1.3

Ipod Touch 2g
2.1.1-4.2.1

Ipod Touch 3g
3.1.1-5.1.1

Ipod Touch 4g
4.1-5.1.1

Iphone
1.0-3.1.3

Iphone 3g
2.0-4.2.1

Iphone 4
4.0-5.1.1

Iphone 4S
5.0-5.1.1