I don't know if it's hacked as much as having an address spoofed. I had a yahoo address that, when i checked mail, had about 20 bounced mails and realized maybe it'd gotten hacked. I changed the password and moved on. nothing else in the box seemed to have been accessed so i really don't know if they got into it or just spoofed it.
I know spammers used to - and probably still do - join yahoo mailing lists and download the whole membership registry, then will use those addresses to either send spam to or as addresses to spoof.
Spamming gets worse around the beginning and end of each month. My theory - all those 'work from home' schemes that are really sending spam, they probably have a monthly quota. The first week's spam is the over achievers, getting thier quota done, the last week's spam is the procrastinators trying to beat the deadline.
I also wonder just how secure yahoo, gmail, hotmail, etc keep thier mail lists. I once had a hotmail that i signed up for, never used, never gave the addy to anyone but it got spam, which suggested to me that either microsoft sold my addy or their general directory got hacked.
There seems to be no end to what these folks will do, and no one seems willing to even try to stop them. And, to a certain extent, why should they? Last I heard, spam mail was about 50-70% of all e-mail out there. Think of all the ad views Yahoo, GMail, Hotmail etc lose if they really blast the spammers (and you can't tell me that any mail with 'c1alis' in the subject doesn't scream spam)
there's no real penalty for the e-mail companies to crack down on spammers. They're their biggest customers.