i know this works in oracle but can't say for sure about MySQL. it may or may not be more efficient, probablly not since it's a subquery, but might be worth checking the execution times if it's important.
select * from table where datefield = (select max(datefield) from table);
Kirk's solution is probablly the optimal one (and the first one I would have suggested) but if you're interested (and it works in MySQL), you might try running some performance tests with both queries.