With iOS 4, the built-in Notes app gets the ability to sync with multiple IMAP accounts, including MobileMe and Gmail. When you set up MobileMe or Gmail (not via Google Sync/ActiveSync/Exchange but via the Gmail button) there's a toggle to turn notes sync on or off.
If you enable notes sync for multiple accounts, you get a new Accounts button at the top left of the Notes app. Tapping it takes you to a new screen where you can choose to view All Notes, just the notes on your iPhone, or just the notes that are synced via IMAP to your email account(s).
Those notes then appear in your desktop and web-based IMAP clients as well, like Apple's Mac Mail.app (in Marker Felt, ugh!) and Gmail.com.
If you're using multiple IMAP Notes accounts, let us know how it's working for you. For more features, see our complete iOS 4 walkthrough.