Actually, there is a 64 bit Oracle provider available from Oracle (OraOLEDB.Oracle.1, see the Connectivity whitepaper).

When implementing ETL packages at a customer however, we found that no performance benefit from using the 64 bit OraOLEDB over 32 bit MSDAORA, so we stuck to MSDAORA, and scheduled our packages to run with the 32 bit DTEXEC instead of the 64 bit.

Once we had all data inside SQL 2005, we ran all other packages on 64 bit however, with considerable performancegains over running 32 bit!