When setting up an email account like Gmail, 10.5's Mail Setup Assistant provided an option to do it automatically or not via a checkbox in the Setup Assistant.
That checkbox is gone in Mail 4.0 in Snow Leopard, and Mail Setup assistant creates such an account automatically. In particular, a Gmail account is always created as IMAP, with seemingly no way to create it as POP.
To bypass the automatic account creation, hold down the Option key after entering your email address and password. Then the Create button will change to Continue, and Mail will allow you to set up the account manually. I should add that I consider the elimination of the checkbox for automatic setup in favor of this method to be an awful design decision that defies logic or explanation.
Mac OS X Hints