Since this thread was started, Oracle 9i Release 2 was made available which also supports data segment compression:
CREATE TABLE table_t$a1 COMPRESS AS
SELECT * FROM table_t
Oracle9i Release 2 (9.2) achieves a good compression ratio in many cases with no special tuning. However, if you need a better compression ratio, tuning can improve it slightly in some cases and very substantially in other cases.
Heap-organized block-level compression works by eliminating column value repetitions within individual blocks. This is done by moving such repeated column values into a shared block-level symbol table and replacing occurrences with references into the symbol table. Therefore, the compression ratio is higher in blocks that have more repeated values. As a database administrator or application developer, you can take advantage of this fact by reorganizing rows in the segments that you want to compress, to increase the likelihood of such repetitions.