I'd say turn on the set in your current cabinet and watch it for a few hours.

If it doesn't get really warm, you're fine.

If it gets too warm, make sure you are getting airflow from those rear vents. ie: make sure the cabinet isn't pushed up against the wall.

If it is still too warm, then get a bigger cabinet.

or get a small fan for the back of cabinet. like what is used on a desktop computer. you could cut a hole in the cabinet back and mount the fan. pretty easy.

Actually I don't see anything wrong with that, we've just purchase a new plasma tv last month and got the same problem too, though I think to be safe and secure it would be better if you consider what was instructed..

Here is a link that might be useful: samsung lcd tv