PG_XLOG grows and grows

Hi all, I have a little problem,

Every once in a while I observe that our filesystem is running out of
space due
to an continous growth of our transaction logs.
After inspecting the backend processes I see that three of the type:
idle in transaction.
And that all of our nightly vacuum runs since the "idle in transaction"
is waiting
with an: VACUUM waiting.

Is there any way of finding out exactly what the "idle in transaction"
transactions
have issued so far? What DB / logins etc. they are associated with?


Regards,

Niclas Gustafsson