I ran across a problem for which I found no documented solution, but inadvertently found the solution myself. So I wanted to document this here for others who may encounter the same problem.
I was using a CAML query within SPServices to retrieve list items, then referencing "ows_{fieldname}" as usual to retrieve the field value for each list item. The field value was reported as "undefined" for all items. I quintuple-checked that I was specifying the proper name of the field and that my query was properly constructed.
I eventually discovered that the fields I was trying to reference were not displayed in the default view of the list. As soon as I changed the list properties to include the fields in the default view, the proper field values were returned within my javascript.
I don't know how much of Sharepoint's underpinnings work, but I was very surprised at my finding because it implies that the SPServices GetListItems method gets its data from the list's default view, not the actual underlying list. I realize I could be wrong in this interpretation, but I imagine others could run into this same problem.