Among other things, they will probably run System Profiler, to get an idea of what condition your system believes itself to be in. They may also run Disk Utility.
If you are worried about all the pirated software, music, and videos on your machine, don't be; they don't have any way to verify that this stuff is pirated, and they're not interested in checking it if you don't advertise its presence. Take any pirated stuff off the Desktop and bury it somewhere deep in your Home folder and you should have no problems. If you're really concerned about something, you can put it on an encrypted disk image, but do not store the password to this disk image in Keychain if you do this. Even if you're running a pirated install of the Tiger preview, then there is still no need to worry; they cannot verify that you did not get it legitimately.
Your best bet is to change the password to something like "apple" before you give it to them, and then immediately change it back once they've given it back to you.
And consider putting your pirated stuff on external drives next time. That way you can simply unplug the drive when you need to take the machine in for repairs.