The person who posted her letter to the forum has caught the attention of a moderator, so I recommend checking in on this thread.
UPDATE (7 PM): A Playfish rep confirms that this offer is not spam and is, indeed, legit.
"This is a legitimate offer that we sent to a random selection of players from throughout the Playfish community. Since Playfish was founded in 2007, we've continually sought feedback and advice from our worldwide player community to help make our games as fun and compelling as they can be.
This particular survey is seeking similar player input but since it requires more time of players we felt it was appropriate to reward them for their assistance."
So, if you get an email with this offer, you can feel free to take this survey and in return, you will be compensated with an $80 Amazon credit.