Within the last few days, Google added IMAP support to Gmail, giving users the option to keep messages on the server with third-party mail clients, rather than downloading via POP3. Thankfully this support is also extended to domains hosted via Gmail as well, so personal e-mail domains are now working beautifully with devices like the iPhone, as well.
Instructions on how to set it up are all over the place for regular Gmail, but the picture here shows the slight configuration differences needed to work with hosted domains.