some people aren't up to speed
i jailbreak iphones and ipod touches locally
the third gen is by far the best when jailbroken or not
if you want it jailbroken then if you get a third gen then hope that it has 3.1.3 or lower
if it has 4.0 then it currently cannot be jailbroken (although a jailbreak is on the horizon)
if you go with a second gen then you will be perfectly fine as long as you do not get a late-2009 MC model. (if you get this then you might as well have got a third gen)
so in short:
third gen is better
second gen MB model can be jailbroken whatever firmware
second gen MC model and third gen can only be jailbroken if they are on 3.1.3 or lower
4.0 jailbreak for second gen MC model and third gen is expected to be released soon
if you need any help with anything (including installous, appsync and anything in cydia such as customization) then just let me know and i will answer all of your questions as fast as i can
~shut
EDIT: oh and first gens are complete crap but they can be jailbroken whatever firmware, but they cant use 4.0