Where does the spoken stress lie in the word "significa", as in "qué significa"? I always hear it stressed on the second "i", but wouldn't that imply that it should be written "signífica", which it doesn't appear to be?
The stress goes to the second-to-last syllable, and hence does not need the diacritical acute mark (´). Significa is a regular verb form of the verb significar. Stress in all infinitives goes in the last syllable, and moves to the second-to-last one in present-indicative-third person singular for regular verbs.
The pronunciation *signífica is a mistake, and may obey an assimilation with the adjective magnífico(a). Also note that the corresponding verb form in Italian sounds very similar and is in fact stressed in the third-to-last syllable (IPA: [si.'ɲi.fi.ca]).