Ideas are cheap. It's the execution that count.
It's a good way to pull PS3 owners closer, but they may need to do something to avoid impending disappointments. Some of the very popular suggestions do not make sense. Who's going to be the messenger with the bad news ? ^_^
People will lose interest quickly, or get impatient, when they see no action done within a short time.
Best is to re-channel their energy elsewhere. The call for community support is a good start.
EDIT:
On second thought, the site may be good for engaging players and surveying their needs (voluntarily). It's useful for market research and customer support, rather than idea sharing.
It's helpful to gather quick data points to break internal deadlocks/debate sometimes (i.e., the "ideas" or topics can also come from the devs). I think it's a good complement to the developers' blog posts.