Wrong output for 7.0.3 from NULL fields in CASE statement

Justin Clift (aa2@bigpond.net.au) reports a bug with a severity of 2
The lower the number the more severe it is.

Short Description
Wrong output for 7.0.3 from NULL fields in CASE statement

Long Description
I am getting output that doesn't make sense from a simple SQL statement. I am expecting to get a '0'::text returned by this statement, but instead I'm getting very, very small numbers, or other weird things. It's a catch for NULL values and isn't interpreting them correctly when it gets them.

I have tested this on a machine with PostgreSQL 7.0.3 compiled from source, and I have tested this on a machine with the postgresql-7.0.3-2mdk RPMS available from ftp.postgresql.org... so this isn't due to the known problems with Mandrake's dodgy 7.0.2 RPM's.

Please see the attached code to see what I mean.

Sample Code
The fields "cashback" and "surcharge" are NULL for this query, but it seems to be returning very small numbers instead.

foobar> select * from transaction_payments;

idnum | payment_type | currency | voucher_idnum | exchange_rate | amount | cashback | surcharge | gst
-------+--------------+----------+---------------+---------------+--------+----------+-----------+-----------
139 | 6 | 18 | | 1 | 170 | | | 17

foobar=# select text(CASE WHEN (cashback = NULL or cashback = 0) THEN '0' ELSE cashback END) from transaction_payments where idnum = 139 and payment_type = 6;
text
-----------------------
1.01855797968803e-312
(1 row)

foobar=# select text(CASE WHEN (surcharge = NULL or surcharge = 0) THEN '0' ELSE surcharge END) from transaction_payments where idnum = 139 and payment_type = 6;
text
-----------------------
1.01855797968803e-312
(1 row)

foobar=# select text(CASE WHEN (surcharge ISNULL or surcharge = 0) THEN '0' ELSE surcharge END) from transaction_payments where idnum = 139 and payment_type = 6;
text
------
NaN
(1 row)

foobar=# select float4(CASE WHEN (cashback = NULL or cashback = 0) THEN '0' ELSE cashback END) from transaction_payments where idnum = 139 and payment_type = 6;
ERROR: Bad float4 input format -- underflow

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