I'm having the same problems. i'm on BIS for Yahoo mail, and BES for corporate email. BES is working fine, emails are coming down in (near)realtime. Yahoo mail is delayed and operating in bursts.
i've deleted the yahoo service books and resent them. i've deleted my yahoo account on my Bold and set it back up again.
problems for me started thursday night - as of this morning, it appears to be okay - but i'm not certain.
i think this must be a yahoo problem. i wish i knew who to contact at yahoo.