For example the following query works fine:
WHERE expires_at <= '2010-10-15 10:00:00';
...this is obviously performing a 'string' comparison
No - if the date/time format matches the supported format, MySQL performs implicit conversion to convert the value to a DATETIME, based on the column it is being compared to. Same thing happens with:
WHERE int_column = '1'
...where the string value of "1" is converted to an INTeger because
int_column's data type is INT, not CHAR/VARCHAR/TEXT.
If you want to explicitly convert the string to a DATETIME, the STR_TO_DATE function would be the best choice:
WHERE expires_at <= STR_TO_DATE('2010-10-15 10:00:00', '%Y-%m-%d %H:%i:%s')