Idle in transaction ????

we've just started using zeos postgresql db components in delphi and i've
noticed that vacuum is hanging in postgres.

if i do ps auxf i get the following

postgres   332  0.0  0.5 13832 1344 ?        S    14:24   0:00
/usr/local/pgsql/bin/./postmaster -i -o -F -B 1024 -N 128
postgres   722  0.0  1.3 14704 3496 ?        S    14:55   0:00  \_ postgres:
postgres ohm 10.139.13.112 idle in transact
postgres   752  0.1  1.8 14580 4740 ?        S    15:04   0:01  \_ postgres:
iain ohm 10.139.13.187 idle
postgres  1086  0.5  1.3 14620 3360 ?        S    15:23   0:00  \_ postgres:
postgres ohm [local] VACUUM waiting

it's the 'idle in transact' that i'm puzzled about and i'm assuming it's
idel within a transaction block ...

can anyone help ???

Steve