ALTER TABLE with USING clause for timestamp

Hi,

I have a column in a table defined as type TEXT and I'd like to
convert to a TIME type - I tried this:

ALTER TABLE t1 ALTER COLUMN tcol TYPE TIME WITHOUT TIME ZONE;

which throws:

ERROR:  column "tcol" cannot be cast to type "pg_catalog.time"

From reading the ALTER TABLE documents there appears to be a USING
clause that I could use to do the cast manually, but the examples are
for converting integers to timestamps and my google-fu isn't giving
me any help.  Any ideas on how to convert from text to time without
a drop/add? The data is in HH:MM:SS format.

PG 8.1

Thanks,

Josh