His last name is Wilkes.

Well, I don't know much about the use of nicknames in GB. Moriarty call himself Jim, so maybe Sebastian just went by Seb when he was younger?

I am not sure about how close they were but I found it so sad to see how Sherlock tried to make Sebastian believe that he was 'normal'. It seemed so out of character for him.

Seb is definitely more informal. Sherlock may have used the more informal nickname as Sebastian is disagreeing with him in order to imply his own superiority in this matter/discussion.

While I am not a great fan of "Blind Banker" as an episode, I do love scenes with Sebastian and the dynamics between Sherlock-Sebastian-John. They are really interesting, to begin with Sebastian calling Sherlock "old buddy" in his letter, then this very telling exchange about "friend" in the office and finally bathroom scene. 
  Sebastian calls Sherlock "buddy", but he doesn't treat him as one - in fact, he keeps humiliating him all the way, by treating him just as a private eye he wants to hire and pay for his services. Sherlock, on his side, is probably not interested in a lasting friendship with Sebastian, but he obviously  would like to be treated by S. as a "collegue", and is hurt by his arrogance - especially in front of Watson. I think "Seb" is his last, somewhow desperate attempt to put their relationship on personal level - that of old collegues from University - and he is quite brutally told off by a wonderfully bitchy Sebastian. Poor Sherlock! 

well, I think Seb is a guy who likes pretending, being jovial and easy-going. kind of a City professional, cold-hearted, rich and without real friends (but pretending everyone is his friend). it seems to me that Seb is trying to show John that he and Sherlock were something like friends at the uni /maybe he´s surprised that Sherlock calls John his friend/, but I don´t think they actually were. they can have some breakfasts together, but Sherlock was too annoying and unfriendly to all. but it was maybe his intelect and oddity that made people around him want to be, or pretend to have been, his friends. just my uncomplete opinion.
btw - are there any ideas which university or department did Sherlock attend? I´d say chemistry or something like that

Somehow I can't really imagine Sherlock being somewhere else than London.
And I really, really want to know what Sherlock did study (chemistry?), but that's getting a bit OT, I think. ;)

Seb will refer to either Oxford or Cambridge with reverence, even if he had gone there himself. These Univesities have the same standing as the Ivy League.

Bizarre theory of the day - seeing the name 'Sebastian', my first thought is of Sebastian Moran, Moriarty's right hand in canon.  Does anyone think it's possible that Sebastian Wilkes is this universe's version of Moran?  In ASiP, the cabbie was meant to attact Sherlock's attention.  In TBB, Sebastian brings Sherlock into the case.

Moriarty tells Sherlock and John that he can't let them live because they're interfering with his business, and TBB would seem to be evidence of that - a random case in which they foil Moriarty even though they don't know he's pulling the strings in the background.  However, there were obviously cases prior to ASiP (and after Carl Powers) in which Sherlock had run up against Moriarty - cases he'd solved - that drew Moriarty's attention.  So TBB could be another test, another manipulation.

Mrs.Wenceslas wrote:

I don´t think so...if that was the intention of Mofftiss, they would name him fully after original, I suppose.

I would tend to agree, but they've renamed at least one other character (Henry Knight).  Then again, there were reasons for that, since Baskerville Hall became a military facility in the BBC story and they dispensed with the family legend part of the story.
 

I think Sebastian and Sherlock have never been friends. Sebastian comes across to me as a controlling jerk. At uni Sherlock was probably the butt of everyone's jokes and was disliked for being "cold" and not having friends or a social life. If you watch their meeting you can see the subtle emphasis Sherlock puts on the word "friend" and Sebastian's incredulous echo. John senses tension right off and settles the matter with "colleague". It's quite like Sherlock is trying to show Sebastian that yes, he is capable of having friends and that he should be taken seriously.

Sebastian treats him in an overly familiar way as if he is trying to smooth down an old disagreement so that he can use Sherlock's abilities to solve his problem. In his email he calls him "buddy" and when he shakes his hand at the bank he clasps Sherlock's wrist with his right hand, the type of handshake someone uses to show intimacy in a professional setting - though if there is no real intimacy in the relationship, it tends to put the recipient of the handshake off. He doesn't shake John's hand this way, keeping his right hand in his pocket. In Sebastian's mind, John is a "normal" stranger and doesn't need to be ingratiated.

Also, when Sebastian is visited by Sherlock at dinner, where he is dominating the conversation with a funny story, he quickly dismisses him to talk to his secretary - another dominant move, showing that he wants to be in control of the situation even though it is obviously an urgent matter. Sherlock insists on telling him about Van Coon's murder and needles him about the appointment. Sebastian briefly looks down and tugs at his collar - he's been made to look bad and doesn't like it.