After having registered a couple of new accounts, here are some notes. Perhaps they could be added to the first post?

Inviting yourself.

On the official web page you can send "invitations" to other players (or yourself).
http://www.wow-europe.com/account/recruit-a-friend.html.
(link for US, other?)

The receiver gets an account with ten days of free play. If you upgrade the account and pay for at least one month, the main will get a free month. Unless you intend to cancel your main account in the next couple of months, this means you get 70 days of game play when you buy the game (including the free month you get when buying the game).

I know you can invite yourself (same name and adress), but I used different email adresses for each account. Has anyone tried using the same email address for different accounts?

About the installation.

If you do a fresh installation on a new computer, you will have to suffer through all the updates. Instead you can copy an existing installation to the new computer.

About multiple installations on the same computer, see here for windows:
http://www.dual-boxing.com/forums2/viewtopic.php?t=1767
and here for mac osx:
http://www.dual-boxing.com/forums2/viewtopic.php?t=2404

Trial accounts and buying the game.

You can play for ten days with free trial accounts. Trial accounts are limited; you can't trade or send mail. You can only reach level 20.
You can't invite people to groups but you can be invited.

If you buy the game, upgrade as soon as you can. You will not lose what's left of your ten free trial days.

When you are ready to buy the game, there are three boxes on the shelves right now:
World of Warcraft (the game)
World of Warcraft: The Burning Crusade (the expansion)
World of Warcraft Battlechest (game plus expansion)

Buying the Battlechest could be cheaper than buying the game and expansion separately (see this post http://www.dual-boxing.com/forums2/viewtopic.php?t=2539), but look around and see what deals you can find. If you're not sure whether to continue playing or not, you might want to wait with the expansion.

In the game box is a "retail authentication key". You log in to account management on the WoW official web page and enter the key.