Unless the Eris has some hidden issue that the Moto Droid does not, your Yahoo mail should work. It just requires a little more setup than Gmail, and more setup than you had on your Storm. There's a thread about setting up Yahoo mail on the Droid- I imagine it applies to the Eris as well.
You just need to set it up manualy as an IMAP server and enter the IMAP and SMTP addresses as noted in the threads/videos. Took me no more than 2 minutes of searching to find the threads, and I think that was because I got sidetracked.
I'm checking Yahoo, Hotmail, and Gmail from the Droid. Yahoo & Hotmail just don't get the mail pushed from the server like the Blackberry does- you have to set the interval at which they will check for new mail.