Where PG might degrade worse than Oracle would be with deletes and updates to the objects table. But these are rare in practice, and inserts should be fine.
I don't personally worry about access times with relatively large tables as long as queries are optimized correctly and indexes used. Remember that access is O(log2(N)) where N is the number of rows. 4 billion rows will require 32 "pokes" at the index to find a row, in other words, not so bad. That's only twice as slow as the number of index "pokes" needed to find a row in a table with 65,536 rows.
The problem we could run into is that some queries in the system might be generating plans that force sequential table scans on acs_objects, and obviously that would be *very* bad. If we run into such queries clearly we need to fix them, though. This would be true even if we didn't make such heavy use of acs_objects.
