They aren't in the Great Hall just yet so no, he hasn't spotted Quirrell (also remember, Voldemort wasn't directly possessing Quirrell until he started 'messing up' and it seemed to intensify over the course of the year; and Tom might not recognise 'himself' in Quirrell immediately, especially if he doesn't know to look. . .) and well, Tom's in Harry's head . . . wouldn't want the Sorting Hat to recognise him and call him out, now would he? ;) (And mayyybe Tom doesn't want to interfere with little Harry's own Sorting to wherever he belongs. Maybe.)