Ok, this is a lot to answer, but I'll do my best on what I know.

Quote:
Is iTunes smart enough to know that files are unavailable if you aren't connected to the share? How does one make some files locally available and the rest on the server?

iTunes checks for the existance of the file only when it tries to play. So, if you add in all the network stuff, and have some local music, all the files show in the iTunes interface when away from the file server. When it attempts to play a file that is not there, it will mark it with an exclaimation point and move on to another file. Once the network is reconnected, trying to play the exclaimation files will cause iTunes to try again and remove the mark. Easiest way to probably work with this would be to drag the music into the lefthand side playlist area and thus have a new playlist created with all the network music. Or, if you have a PC somewhere on your network always on, run iTunes on it, add all the network files, and use the iTunes sharing feature to then access it on the Powerbook. Since you said you have a Linux server, mt-daapd is worth a look. It will let the Linux server share using the iTunes protocal.

Quote:
Will iTunes make use of Album cover art in any way? If so, what are the requirements?


Album art must be in the tag in the proper id2.2 or higher format. To see the album art in iTunes, click the button on the bottom left area that has a bar and a triangle pointing up. That reveals the album art viewer. Clicking the words "Now Playing" will toggle to "Selected Song" and vice versa. Selected song does not work with the shared iTunes music protocal, but now playing does.

Quote:
2. How can I have the powerbook automatically mount network shares when joining that network?

Once the share is mounted, open the Accounts system preference, and go to Startup Items. Drag the network share there.

Quote:
I don't want to have to wait forever for it to time out if I'm connected to another network.

I believe it will try and connect, but not block startup when it can't find the server. I use netatalk 2.0 personally for my Powerbook shares, as for some reason my samba server and Powerbook don't like each other very much, giving odd errors like the filename is too long, even with a 1.1 filename. It's only my server, and ones else work fine, so It's not the Powerbook.

Quote:
If I'm plugged-in to the ethernet, and then disconnect, is there any way to make the wireless just take over the connection?

This is one thing I really wish Windows would do. The Mac will seamlessly transition between wireless and wired as long as both interfaces have the same IP. Either create a new network location and set static IPs for both, or tell your DHCP server to assign the same IP to both the wireless and wired MAC address. I do this all the time at home. If I do anything like a big file transfer, I'll start it while on wireless, then find the cable, plug it in, and watch the Mac choose the faster wired interface.

Quote:
Do you guys have some favorite sites or forums for OS X technical information?

www.MacNN.com for news, www.MacOSXHints.com for some nice tips. www.versiontracker.com is good for software. Album Art wise, Clutter might be worth a look. While I find it's displaying of album art on the desktop pointless, it does also offer a feature to grab album art from Amazon.com if the current track in iTunes is missing it. Tagging wise, I haven't done a bunch in iTunes, but the interface seems nice enough. Most of my tagging was done so long ago, I did it with tools on the PC.