The Astro File Mananger a free app on the Android Market enables you to backup your apps and appears also lets you backup paid apps BEFORE the refund period is over. Quite a large flaw in the refund scheme used in the Android Market, will keep testing as a few apps don't seem to be backupable.
1. Install Astro File Manager.
2. Purchase app.
3. In tools, application manager, backup check 'backup'.
3. Refund app.
4. The .apk install file is still in the backup directory for installing again, after the refund.
5. Free paid apps.
edit: Seems the apps that won't backup are private apps. Googling suggests these can be backed up on rooted devices.