Setting up an email account is not always straight forward. Apple, with each Mac OS X release, has attempted to streamline the process offering automatic account setups. For some users, however, this poses an issue with control.
In Snow Leopard, when attempting to setup some accounts, such as a new Gmail account, the automatic setup provided by Apple sets the account up as IMAP by default. Some users still prefer POP Accounts and it is not immediately clear how to restrain Mail from setting up the IMAP default.
When setting up a new account, enter your email address and password, but before pressing create, press and hold the Option key (you may have to click outside the field boxes to enable this function). You will notice the "Create" button becomes "Continue", as below:
Once you press "Continue" you will be presented with options for IMAP, POP, and Exchange information, as well as customization options for your incoming, and later, outgoing mail servers. You can continue holding the Option key throughout the setup process to customize each step or press "Create" at any point to have Mail attempt to automate the remainder of the setup process.