Transaction isolation and UNION queries

I have a question about transaction isolation that I can't find an
answer to in the docs.  I'm working with a database that has some data
split over two tables.  One table is the ultimate destination for all
the data, the other is a "pending" table which holds rows during data
entry.  Rows from the pending table are moved to the permanent table
once data entry is complete.  For some purposes I want to see rows from
both tables, so I do a UNION.  My question is, with only read committed
isolation, could a commit by another transaction make changes appear
between the separate parts of the UNION query?  In other words, could a
row appear to be missing or duplicated because a transaction that was
moving the row from pending to permanent committed while the UNION was
running?

Thanks!

Bob Smith
Hammett & Edison, Inc.
bsmith@h-e.com