Hello Tracy, Yes, heavy tile can be installed onto walls and even fixed to ceilings, if you like. The key however is in the preparation. You should have adequate anchors and support for your underlayment before you specify either. Gypsum wallboard should not be used under these tiles.
Rebecca is incorrect however about the thinset required. A "large format" mortar is simply one which is formulated to resist shrinking while it cures and compression from the mass of the larger tiles. While this is important for floor installations, walls and ceilings depend more on adhesive bond strength. For walls or ceilings, specify that your installer use a "non-sag" formula, which most often will be appropriate for large format tile anyway. For installations over impermeable membranes, such as used in steam showers, a "non-sag rapid-setting" formula should be used instead. I prefer the Laticrete product line, and more specifically their "4XLT" mortar for walls and ceilings. I've been using their new "4XLT-Rapid" in the steam showers I've built lately and I like it's properties very much.
Best of luck,
Shaughnn