I've got CDs on there which I'd ripped onto my old iMac but weren't purchased from iTunes. My old iMac was burgled so I now want to sync my iPad with the replacement. When I tried to sync it I got a message saying the contents of my iPad would be erased and replaced with the ilbrary on the computer, bu that only has the stuff I've purchased from iTunes. I've not managed to update to iOS 5 yet as I haven't synced as I don't want to lose everything on there.
If you've authorised your iTunes account(s) on your computer's iTunes then you should be able to copy iTunes purchases via File > Transfer Purchases. For non-iTunes purchased music, if you haven't got a backup of content on external drives, CDs, DVDs etc, then you will need to see if there are any third-party programs that can copy content from your iPad to your computer