Yahoo! Mail doesn't work via wireless because your device's IP address becomes your home internet address and not one of the Verizon addresses that are allowed to use the IMAP/POP service for free.
"Seven" beta is an awesome program for Yahoo! Mail and kept me from switching to gmail (been with Yahoo since 1999 and not a big gmail fan for mail). It does "push" to the device, keeping your phone from chewing up its battery by polling yahoo servers. Register for it here:
SEVEN Beta Program :: Welcome to SEVEN Beta Program
Then, DO NOT try the download link because our device is not listed. Instead just login and then use this link and install as you would any .apk:
http://community.seven.com/files/Z7-rel23-384237-def_seven-preview.apk
You'll love it. Includes an IM client too, for Google chat and Windows Live, but I don't use that. I use the email program for my yahoo and gmail.
I've attached some screenshots.