If I put a CDROM in my CDRW device, I don't seem to have trouble accessing the data (mp3's, video's, etc...)

However, when I am done and want to remove the CD, pushing the eject button doesn't work, ejecting through KDE doesn't work and unmounting doesn't work.

The errors tell me that ejecting failed and that the device is busy.

So, how do I know what apps are using my CD-ROM so that I can try shutting them down in order to get my CD back?

The only solution I've found so far is rebooting, which isn't desirable, obviously.

ps ax |grep /dev/hdc

this assumes your drive is mounted as hdc. Please change accordingly.