So is every other enclosure...minus Apples. Also dont tighten the Maxupgrades optibay too much or you will bend the speaker grill on top.
I don't think it matters very much but my Newmodeus SATA-SATA is made from metal. It's not like the thing is under a lot of stress in there though. Plastic should be just fine. Might be lighter, even.
An update on OS X 10.6.5 / 10.6.6 troubles I had - my problems were due to an 8GB RAM update I had gotten from OWC, not due to the SSD. Beware of OWC RAM.