Optimizer ignores datetime index

[Angus]

I have this very simple query which I think should use a certain index on a DATETIME column, but does not:
explain SELECT * FROM t1 WHERE Start<NOW()

The column Start has its own index. The explain says that the "type" is "ALL", and correctly identifies the relevant key under "possible_keys", but then doesn't use it.

According to what I've read, the optimizer dispenses with indexes if it thinks it will read a large percentage of the table. The SELECT should return about half the rows. I've also heard that constants need to be used for an index to work. The manual says that NOW() does return a constant, but if I change it for Start<'2006-10-27' this doesn't cause the index to be used either.

USE INDEX doesn't cause the index to be used, only FORCE INDEX does. This suggests to me that the optimizer knows something that I don't, and FORCE INDEX is suboptimal.

The database is InnoDB and the server is 5.1