Query with rightmost function does not use index

For a table where column col has an index, the query:
     SELECT ... FROM ... WHERE col = MD5('')
doesn't seem to use an index but
     SELECT ... FROM ... WHERE col = 'd41d8cd98f00b204e9800998ecf8427e'
does.

Is this a gotcha?
I'm using PostgreSQL 8.4.8 on i686-pc-linux-gnu, compiled by GCC gcc-4.5.real (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2, 32-bit