Hi Gina,
Yes it is a limpet. I had heard that they can sometimes have very sharp rasping mouth parts, so keep an eye on them in case you're using an acrylic tank.
Anthony Calfo spoke a bit about Limpets and Chitons at our local meeting a couple weeks ago also. He said that Chitons are almost all harmless and completely safe to have in our tanks. Limpets though can be not-reefsafe. Basically if it's grey to brown or not highly colored then it's likely safe. Any brightly colored limpets may be predatory or cause trouble. His words, "You can't be big, orange, squishy and be able to sit out in the open unless you have a good defense."
The one you've pictured looks just like ones that appeared in one of my tanks, lasted for quite a few years, then all disappeared. I doubt that it would cause any problems.
-Rogue
