Gmail also seems to shorten the names of people you email to (at least for me). That makes me second-guess whether I'm really emailing the right person at times.
It always shows last names for me (so long as they're available). Unless you've got some new feature, I'm guessing those contacts just don't have full names assigned. Spending some time in the "contacts" tab, cleaning things up really made things easier for me in this respect.
