This turns out to not be possible to do with a formula, but there is a button on the "ribbon" labelled "Conditional Formatting" which does this (once). This button allows formatting by equal to, ranges, and even not-equal to although that is not one of the first options provided.
Here's the terse StackOverflow answer that broke this open for me initiallly:
http://stackoverflow.com/questions/5194286/highlight-cells-in-a-row-where-the-value-of-a-particular-cell-is-different-to-th
Here it is for an earliere version of Excel:
http://www.timeatlas.com/5_minute_tips/general/how_to_use_conditional_formatting_to_highlight_data
