If I request coordinates for St. John's Newfoundland: http://local.yahooapis.com/MapsService/V1/...ns+newfoundland (preview shows above URL truncated - after ? I specify appid=whatever followed by &location=st.+johns+newfoundland). If necessary, I can supply my true appid. I get reasonable results when I omit the apostrophe (latitude 47 longitude -52). However, when I properly include the apostrophe in the name (whether directly or as %27), the result returned is for St. John, Pennsylvania (41 -75). Similarly, Martha's Vineyard spelled correctly returns Vineyard, Utah, but spelled without the apostrophe returns the correct coordinates. Same peculiarity for Coeur D'Alene, Idaho, Val d'isere, France, etc.
Specifying an apostrophe does NOT confuse the Yahoo Maps website (http://maps.yahoo.com); It locates St. John's, Martha's Vineyard, etc. just fine. It is only when using Yahoo Maps API that confusion seems to set in.