Binding fields in the registration form

[jbenito]

Enable/disable or display/hide fields depending on the value of another field.
E.g.

http://www.hecr.tifr.res.in/MaKaC/c... 
We did get one feedback on our form - if one chooses "I don't need 
accommodation" can one automatically disable the hotels section? Or, 
for that matter, in the Personal Data, if the country selected is some 
X (India in our case), can I automatically disable asking for extra 
details (like passport details for foreigners), the second field in 
our form.