From a visual standpoint there could be a David and Goliath effect produced, that is a conflict of scale as the maple is liable to have a tree-like habit as well. Trees are accents that set the scale of the scene they inhabit.

The silver maples in my yard don't seem to mind the walnuts at all.

Not anywhere near a walnut but my old crimson king is maybe 200 feet from the nearest group of walnuts and its fine.

Check this.
I await the wisdom of others concerning its correctness.
http://www.uwex.edu/ces/news/cenews.cfm?ID=2135

Notes, I didn't make the connection on the url you listed.

The one below gives info that I have heard before and thought was well known. Although I don't have a first hand experience.

Here is a link that might be useful: Maybe this one?

I don't think PIM was talking about visual comparisons. The allelopathic properties of black walnut (Juglans nigra) have long been known, but not all plants are affected, and those that are, are affected to varying degrees. Most Acer species are unaffected or only marginally affected by the allelopathic affects of the juglone associated with black walnut, but that is not to say they won't suffer from root competition.

Al