Listing all open sessions/connections/XAs ?

Hello,

Occasionally I see that my (web) app leaves some DB connections open,
so they look like this to `ps':

  postgres: username dbname 127.0.0.1 idle in transaction

This results in my DB connection pool getting exhausted every so often.
I need to track the source of this problem.

Is there a way to see the SQL (or any other information) associated
with a connection/session/transaction that caused my DB connection to
remain open?

I know MS SQL Server has something like that, and so does Oracle, but I
haven't seen this mentioned in the PostgreSQL Admin Guide.

I have PG 7.3.4.

Thanks,
Otis