A silver tip roast is definitely not a tri-tip, silver tip is cut from somewhere around the shoulder, IIRC, while a tri-tip is from the other end of the animal at the bottom end of the sirloin.
Wikipedia has some alternate names like culotte and triangle steak, but those are the names of steaks made from the tri-tip, not the whole thing. I've heard "triangle tip" but I think that was from someone who was confused. If you're on speaking terms with your butcher, the cut classification for him/her to look up is: NAMP 185D
Here's the wikipedia page:
http://en.wikipedia.org/wiki/Tri-tip
Here's a pretty prototypical photo of what one looks like (scroll down):
http://www.foodsubs.com/MeatBeefLoin.html
Should be relatively easy to identify if it's packed by itself. They're typically shaped somewhere between a triangle and a capital "L". I've seen photos that people posted of tip they bought back east that were obviously not tri-tip, but probably just sirloin cut into a triangle shape and labelled disingenuously.
Price of tri-tip isn't quite as high as it was a few years ago when it got crazy popular all of a sudden, but it's still not as cheap as it was when everyone thought it was garbage meat. If it's priced higher than new york or rib roast, skip the tip and buy those. Don't buy too many of them, there's only two per steer! Think of the children!
