Transaction IDs wrap around, per my proposal of 13-Aug-01. More

doc/src/sgml/catalogs.sgml

doc/src/sgml/ref/create_database.sgml

doc/src/sgml/ref/vacuum.sgml

src/backend/access/transam/clog.c

src/backend/access/transam/transam.c

src/backend/access/transam/xid.c

src/backend/catalog/genbki.sh

src/backend/catalog/index.c

src/backend/commands/dbcommands.c

src/backend/commands/vacuum.c

src/backend/commands/vacuumlazy.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/backend/parser/keywords.c

src/backend/storage/ipc/sinval.c

src/backend/utils/time/tqual.c

src/bin/initdb/initdb.sh

src/include/access/transam.h

src/include/access/xact.h