8.2.4 Won't Build 8.1 Functional Indexes

I am testing upgrades to 8.2.4 from 8.1.3 and am having problems with
8.2.4balking at the functional indexes I have created. These indexes
exist and
work fine in 8.1.3, so why is 8.2.4 rejecting them?

Index 1:
CREATE INDEX acceptedbilling_to_date_accepted_billing_dt_idx
ON acceptedbilling
USING btree
(to_date(accepted_billing_dt::text, 'yyyymmdd'::text));.

Reject:
ERROR: functions in index expression must be marked IMMUTABLE
SQL state: 42P17

Index 2:
CREATE INDEX header_835_billing_idx
ON ct_835.header_835
USING btree
(to_date(to_char(create_timestamp, 'yyyy-mm-dd'::text),
'yyyy-mm-dd'::text));

Reject:
ERROR: functions in index expression must be marked IMMUTABLE
SQL state: 42P17

This is on 8.2.4 installed from the official RPMS.

thanks,

Chris
RH AS 4
PG 8.2.4 from RPMS