The easiest option (which I'm using atm) is to sign up for Nokia Messaging - http://email.nokia.com Then add Yahoo email. On N900, add Nokia Messaging. Nokia Messaging will do all the settings stuff. Apparently there's a unique IMAP server address for Nokia that Yahoo provided. All you have to do is type in your Yahoo email + password like I did.
The imagination consoles people about what they cannot be and the humor about what they actually are. -Albert Camus