I am a Yahoo! Mail Plus subscriber and recently noticed that my outbound e-mail rewrote my name as Philip O'Toole.
Seemingly it had been like this for weeks, but it only caught my attention when I sent an e-mail to my work account. Of course, if you’re a programmer it’s pretty obvious what is going on here.

Yahoo!’s code is replacing the apostrophe in my name with the corresponding ASCII code. I tried resetting my name in the Options->General section but it didn’t help. Finally, after setting up a second free Yahoo! account, I worked out where to fix this – under Options->Account. It is there you’ve got to tweak your name settings.

This drove me nuts – I was even on to Yahoo! support. This might help someone else out there. I would guess that Yahoo! processed the data on their servers recently, and mangled the names while doing so.
I’ve had various problems with Yahoo! Mail over the last year – perhaps because Yahoo! seems to be a company in turmoil, and is having a hard time keeping it together.