The iPhone makes heavy use of email. It's the easiest way to send yourself a URL and the only way to get Notes out of the iPhone (for now, at least).

I generally try to filter as much as possible out of my INBOX, including mail from me (mostly replies to mailing lists I'm on, and I don't need to read what I just wrote), but I want mail from my iPhone to stay in the INBOX where it's easy to find. I was annoyed that my sieve filters apparently cannot match on the body of messages, only on headers, but the solution turns out to be very easy -- I put this at the top of my sieve file, and now mail from my iPhone (but not my Macs) shows up in INBOX:

# personal short circuit
if allof
  header :contains "From" "pepper@",
  header :contains "X-Mailer" "iPhone Mail"
 { stop; }