There exists a small bug in Adium, which apparently has been fixed in an upcoming version, which means that hosted Gmail accounts cannot sign in as GTalk accounts.
To fix it, I needed to create a new Jabber account, with the following settings:
Account.
Jabber ID: matt@schinckel.net
Password: ••••••••••••••••
Options.
Connect Server: talk.google.com
Resource: Adium
Port: 5223
☑ Force old-style SSL
Then it connected fine, first time.
