The following is taken from a post over at The Unofficial Apple Weblog;

1. Yes, it will boot headless, meaning with no display or video device connected, enabling you to have what I like to call an iServe.
2. While it is strongly recommended that you only have an Apple Authorized Service Provider crack it open and install RAM, hard drives, Airport and Bluetooth, it will NOT void your warranty if you do it yourself. As is standard operating procedure, however, anything you break while attempting anything on your own is not Apple’s responsibility and will not be covered under warranty. I think that is pretty much common sense.
3. Airport and Bluetooth can, in fact, be added after purchase. AirPort Extreme card and Bluetooth module attach to the Mac mini’s motherboard via a special connector and will be sold together as a kit for $129.
4. RAM is the most accessible upgrade once you get the case off. That much is clear from the picture.
5. All upgrades other than RAM are not as accessible, but accessing them won’t void your warranty, with list item #2 above in mind.
6. The reason the TOP of the Mac mini doesn’t glow and pulse (this is my only gripe so far) is that there wasn’t enough room to light it once the optical drive went in

I wouldn’t exactly recommend that you tell Apple tech support that you cracked the box to upgrade the memory; phone support people start denying service when they hear that sort of thing. I have a feeling that one of the first after market modifications to these things is going to be the “how to install an LED under the hood” hack.

There’s also a site that has nice shots of a mini Mac vivisection.

I’m thinking a bit about making a pitch to use these boxes on the desktop at one of my client locations. I’m not sure how well it would go over, especially since they’re Outlook addicts.