I have used MailTones for my phone since I got my iPhone. It works flawlessly... but MAY pose a security threat, depending on what you need it for. It works by receiving your email message for you -- you must forward the email to their server; once they get the email that matches your configuration settings, it sends the alert back to your phone.
It's not an issue for me because the emails that I need notification for contain only an 8 digit problem ticket number that I get from the automated paging system at work. When I get that, I know where to go and what to do with that number. If, however, you are having full-blown messages sent to you that contain confidential information (that's a whole other discussion, BTW), the content of that email message is also sent, in its entirety, to the MailTones servers where it could be read by unknown persons.
Just a word of warning... and, as I said, for the right application, it's perfect and works flawlessly.