Hi Andy,
Like Sam Joe's question the other day, I think this is an acceptable topic, although it is also close to the line.
[side note]I am going through the opposite argument at work at present: one of the other people wants to only have a web enabled J2EE application, and I am pushing to have that plus a fat J2SE stand alone application

[/side note]
I think you could get into trouble if you decide that the client's requirement is not required. As Levent said, you cannot discuss this with the analyst or with the client. They may have some reason for wanting the fat client that they just haven't mentioned.
One of the measurables that I have seen mentioned, for both this and the
SCJD assignment, is the ability to follow the instructions, even if you don't agree with them. In giving us the certification, Sun are, in effect, telling potential employers that we are architects. They don't really want to be saying that someone is a good architect if they ignored some of the requirements.
End result: my feeling is that
you should support the fat clients.
Regards, Andrew