The mac mini server comes with NO optical drive. In its place is a second 500gb hard drive. (this may be an option, to replace the ODD with HDD #2, I am not a sales expert, I just fix 'em)
When you boot it up you will see Macintosh HD and Second HD on the desktop. if you wish to raid it, you will need to boot off something (external hard drive most likely, since there's no optical drive) and open disk utility. From there you can either select the main drive, change it to a degraded raid, and then tell it to adopt the other drive as the second slice, or you can simply nuke both drives and immediately set up a mirror.
10.4 at least (and possibly 10.5) had problems with creating a bootable raid that worked properly by creating it from scratch, and required you to create a degraded raid and then fix it. It may be fixed in 10.5 and I surely hope it's fixed in 10.6. I just know I had to do it "the other way" on three occasions, and there's a few articles scattered around that discuss the problem and how to fix it. (both in terminal for 10.4 and in DU for 10.5)
The "Second HD" can also just be used to host server data such as sharepoints, home folders, etc, and not be used for raiding. Apple recommends storing OS and data on separate drives, and I'm in agreement with them on this. But then I'm also in agreement that raiding your data is a very good idea, and raiding your OS is also useful, so you can't have your cake and eat it too.
And then we get into the whole "raid is not a backup" discussion...
