index usage

[Ben <bench@xxxxxxxxxxxxxxx>]

I have a table with 37000 rows, an integer column, and an index on that 
column. I've got a function that returns an integer. When I do a select 
where I restrict that column to being equal to a static number, explain 
tells me the index will be used. When I do the same thing but use the 
function instead of a static number, explain shows me a full scan on the 
table.

I must be missing something, because my understanding is that the function 
will be evaluated once for the statement and then collapsed into a static 
number for the filtering. But the results of the explain seem to imply 
that's not the case....?