Select unique vals in column. Anything faster than SELECT DISTINCT?

[Blah Duh]

I have large table (185 million records, growing at 1-2 million per month). I have already indexed most of the fields, but that isn't helping me with selecting unique values from a column. The query is pretty basic:

SELECT DISTINCT(`FILE_ID`) FROM tableName;

"FILE_ID" is an int(11) value, and is indexed. Doing a "SELECT count(`FILE_ID`) FROM tableName" completes in under a second.

The problem is that the "DISTINCT" query takes AGES to complete. I don't actually know how long, because I stopped the query after 25 minutes. Is there a faster way for me to get unique values from a column?