This suggestion may be a bit un-Arc, but I'd prefer a "with-infix" macro implementing a well-defined sub-language than I would some kind of integration into the reader. An optional precedence list arg might be a good idea.
The problem there is, of course, that the s-exprs generated by the macro are essentially a black box, but in my thinking, if infix is being used minimally and purely for convenience's sake, it isn't necessary to make those s-exprs accessible to other macros (right?...).
