If your application is client-server and doesn't run on the database server, then I doubt you'll see any difference in the application. From a DBA perspective, you will have some differences in how to configure the operating system for shared memory, et cetera. Once the binaries are installed and the database created, it should be very similar to manage.
Since you're doing a platform change, you'll have to use export/import to move the database. If you have a large database, you may consider upgrading to 10g on HPUX and then using transportable tablespaces to move the database to 10g on AIX (transportable tablespaces are cross-platform starting with 10g).
This was first published in December 2004