It fails with both wifi and 3G, I live in Illinois.
I checked my Yahoo acount - just the simple free one, not Mail Plus, and I didn't see anything relevant in any of the Options.
I also restored the iPhone software, that didn't change anything either.
I've never had Yahoo Mail Plus, and all has been working fine for 5 years, I've been on the iPhone 4 since last Summer - one never needed Mail Plus for the iPhone before, and I think they would've listed it necessary with in iPhone when they listed Eudora, Thunderbird, etc. That's not the issue.
Whatever happeneded, happened sometime Friday afternoon, there was no unusual behavior from me that day, I can access the email received before that, and it does see that there are new messages to download, and it does try to download: it will flash "Downloading 1 of 13 Messages" with the status bar, like it would normally, but as soon as you see that, it dispears and goes back to the status quo.
I sent a mesage to Yahoo, looking forward to their reply.