SyncBook
Easily sync important notes between your Mac and iPhone or iPod touch then edit them on either device.
SyncBook allows you to keep all your important information perfectly in sync between your iPhone (or iPod touch) and your Mac, while still being able to edit it on either.
SyncBook allows you to keep and edit plain text notes on your iPhone.
SyncBook’s Mac client allows you to sync these notes to and from your Mac. On your Mac, you can use your preferred text editor to edit your notes.

Download via Apple App Store
Apple App StoreCompatible with iPhone, iPod touch, and iPad. Requires iPhone OS 3.0 or later.
Downloading the free Mac client to get started (requires 10.5 Leopard or later & WiFi).
What people are saying:
We've been waiting for this for SO long... Here it is -- notes that you can sync between your Mac and your iPhone WITHOUT having to store your sensitive data on some web server out there.User, iTunes
It provides the same functionality I had on my Palm and its Palm Desktop software, thus allowing me to fully transition to the Touch as my PDA and toss the old Palm...The support has been great, too. The memos sync over wifi, and I had a problem getting set up because of my unusual and unintentional network settings (my fault, not SyncBook's!). I wrote to the developer, Ross Tulloch, and he wrote back to me within three hours and gave me a working solution.User, iTunes
Great app! Does just what it says--which should be a given--but unfortunately is not for all apps. SyncBook is perfect for those wishing to keep memos that you've created on your Mac or iPhone in sync with the other.User, iTunes