For those that doesn't get it instantly, this means that you may access your email using Thunderbird at home, Outlook at work (you must have a shitty job!), The Bat! at school and the very good Gmail web interface for everywhere else. Everything and everywhere will be synched: If you get an email and read it while at work, it will also pop up (as read) when you open up your Thunderbird at home, in the folder you moved it to when at work.
Many standard users choose to only use the Gmail web access system, as that is very simple to understand yet very flexible and powerful.
Google have choose to handle the tag/folder problem in the most obvious way: if you copy an email to another folder, it will still be the same email, just "tagged" with the new folder. This means that even if you remove an email from every IMAP folder, it will still show in the All mail folder. However, if you move it to Thrash or Spam, it will only reside there, and if you remove it from those, it'll be gone forever.
It was a breeze to set up in Thunderbird, everything just as expected.
For the ones that would get cracking: enable IMAP in the Gmail Settings (and click Save Settings!). The IMAP server is the obvious
imap.gmail.com, using SSL (port 993, standard), and SMTP server, if you want your outgoing mail to go through Google instead of your ISP or your job or whatever, is the obvious
smtp.gmail.com, using TLS. Since many ISPs block port 25 outgoing, thus hindering you from using another SMTP server than the ISPs' own, Google's SMTP server is also available at port 587. (The reason for blocking port 25 are good - it stops evil hackers from using zombie computers as spam servers).
Actually, using the Google SMTP server is a great option even if you don't use Gmail actively (you'll have to sign up for Gmail to be able to authenticate with the SMTP server, but you don't really need to use the actual Gmail). One reason is that the "outgoing email server settings" won't change if moving a laptop between home, work or a random coffee shop. Also, Google Mail's SMTP services implement DomainKeys and DKIM (same thing, really), so that your emails will get a huge "good score" on most spam-filtration systems on the reception side (your email won't end up in the SPAM folder of the recipient).
I will now change to using Gmail as mail host for my way to many accounts and folders, putting nearly my entire life in their hands..! (I already use Google Calendar!).