I tested this by updating one of my domains registrant contact details with my registrar 1and1 on friday. My 1and1 control panel still says the details are updating and when I do a whois it is still set to my old address.
I've also updated another of my domains with the nominet online services, the whois updated straight away, but the address with 1and1 (my registrar) is still set to the old address.
There also doesn't seem to be an easy way to change my address on all my domains either via nominet or via my registrar. So it looks like I will have to phone my registrar to ask them if there is a way that they can update them all at once.