My understanding is that the term Brickwall Limiting means that once the signal hits the threshold it is stopped dead right there and the limiter does not allow it to exceed that threshold. A compressor allows it to go above the threshold to varying degrees based on the ratio.
A good plug in in Sonar to see this in action is the Sonitus Compress. You set it to it's extreme and you'll see the graph show a straight line at the top indicating nothing is being let through above that level (and I think the interface actually changes to say Limiter somewhere).
And in Sonar X3 Producer you get the Blue Tubes Brickwall Limiter. It's pretty nice. There is also the Boost 11 limiter (I think it is still included) but it is harsher sounding to my ears.
The one that most people seem to like and I agree now that I own it is the Cakewalk Concrete Limiter but you have to buy that separately. You just missed a sale on it.
Cheers.