I just got a WD 500GB drive for my 13" MBP and it has the clicking problem. It is only noticeable in a quiet room; if you're watching TV and doing stuff on the MBP at the same time, I don't hear it.
I have worked out that the first minute after not using any programs at all, the clicking will occur approximately once every 12 seconds and then after that it stops altogether.
Also, the clicking becomes much less frequent when you are using an application that constantly accesses the hard drive either to read or write data. Even typing in a program such as Word or Pages is enough to prevent the clicking sound.
So although it would have been nice to have gotten a perfectly quiet hard drive, I can live with the fact that it's a problem that is relatively minor and there are certainly more pressing issues in life to worry about!
(Also with a planned future upgrade to a SSD, the problem will go away, and the current HD will become an external backup drive.)
Edit: On the other hand, all those clicks may well be greatly shortening the life of the hard drive, so it is a concern. There is talk that Snow Leopard may improve power management and there is hope that all these clicking issues will go away then, though I won't hold my breath.