Why ext3 doesn't need defragmentation ?

[Bruno Costacurta <pubmb.bco@pt.lu>]

Hello,
it appears that ext3 doesn't need a real defragmentation operation (by 'real' 
I mean a specific tool that need to be run sometimes related to disk usage).

Is it correct ?
If so how it works ?
Related to ext3 standard setup (see tune2fs) the disk is check by default 
every 30 boots or 30 days. Is this operation includeing some defragmentation ?

Note: by the way I think the filesystem present in Vista still need to be 
defrag...well, once again a feature that was announced but not delivered by 
Bill (please do not hesitate to correct me on these assumptions as I'm not a 
user of such $$$ product). 

Thanks for your attention.
Bye,
Bruno