Of course there are resource limits. "Unlimited growth" is a reasonable thing to talk about *now* because of two facts, below. So, yes, we're limited in our physical resources, but until we are using them maximally efficiently, there's lots of room for economic growth. It may be debatable how long we have until we start to hit physical limits, but it is very likely not this year, nor 10 years from now, nor even 30 years from now. (Look up world population in Wikipedia for projections.)
1. Economic activity is proportional to population size (usually). So as long as the world's population keeps growing, the economy can keep growing. (And as an aside, I have always interpreted economists' talk about growth to mean: "keeping up with the population". Doing so makes all the talk about growth seem more reasonable.)
2. Economic activity, and hence growth, is not only a measure the use of resources, but a measure of the efficiency with which we use those resources. We have lots of room for increased efficiency. One real-world example is the vast underutilization of a vast portion of the world's population.
You can even have economic growth without any increase in physical resource usage. Here's a toy example. Suppose you lived in a world with zero population growth, and one in which every person renewably grew their own food and renewably built their own shelter, with no other economic activity. Now imagine this community discovered music (and at the same time money!). There is lots of room for economic growth as people learn to entertain themselves and others, and as they get better and better at it, and people are willing to pay more to hear the better performers. Then one of the really good performers realizes that it'd better if he pays some bad performer to take over the job of growing his food and building/maintaining his shelter, and the bad performer realizes she's better at that than making music. More growth. So you have lots and lots of room for economic growth even if people are only shuffling around into jobs at which they are getting better in lieu of jobs they're not so good at.