Ok, so far so good.

I've sent an email to possible yahoo ID combinations and I got the below reply:

(first - my first name, last - my last name)

Message from yahoo.com. Unable to deliver message to the following address(es).

<first.last@yahoo.com>: This user doesn't have a yahoo.com account (first.last@yahoo.com) [0]

<first_last@yahoo.com>: Sorry your message to first_last@yahoo.com cannot be delivered. This account has been disabled or discontinued [#103].

<firstlast@yahoo.com>: Sorry your message to firstlast@yahoo.com cannot be delivered. This account has been disabled or discontinued [#102].

<lastfirst@yahoo.com>: Sorry your message to lastfirst@yahoo.com cannot be delivered. This account has been disabled or discontinued [#102].

I felt lucky today. Went straight to yahoo sign-up to get my yahoo ID containing my name. Surprise: This ID is not available.

So...what?