Windows live hotmail on iPhone
The priority for accessing any email service would be:


Exchange: Push email, folders, syncs read/unread status, contact sync, calendar sync
IMAP: Folders, syncs read/unread status
POP3: Bare bones email reading, usually only useful for a single client unless you "keep a copy on the server"


It seems that some people have had success using Exchange; IMAP seems to require either a premium account or 3rd party software; and POP3 is possible for free accounts.

Keep an eye on Apple's page as they will probably update it as new solutions are added by Microsoft.