We went not long after they opened, and the walk-up wait was at least an hour at dinner time. I'd say call directly and get a reservation - it can't hurt to have one if you have an idea when you will be going!

If you haven't seen the posts about this, try and call T-Rex directly if you can't get a ressie through the WDW site. I did this for out trip - no availability on the web site, but any time I wanted when I called.