We use the backup facility shipped with Portal Server (MSDMback) to create an image of the entire Portal onto a network share. This runs on a scheduled task which kicks off a little batch file at 1am every morning. We then pull this onto tape using Arcserve at 4am.
This suits us as the crawls stop while the backup facility is running, and our Portal only needs to be operational during office hours. However, if yours is a 24 hour Portal and it's important to you that content appears immediately, this may not work for you.
Additionally, you'll need to run the backup utility using the cscript command (cscript MSDMback /r etc...), otherwise it will prompt you to click OK to begin the backup routine, which is obviously no good if you're running it on a task.
The disadvantage of this system is that it is not possible to restore individual files, you have to restore the whole image. It's OK for disaster recovery purposes though. For restoring individual files and folders there is a web part called "undelete" which you can find on the Portal Server web site under Downloads.
Hope this is of help.