It's the usual, highly informative, "long-term contract for an undisclosed fee". Arsene Wenger is quoted as saying, "We're delighted to welcome Abou to the Club. He is a strong midfielder and has shown huge potential during his time at Auxerre. He will be a tremendous addition to our squad" and the player himself says, "I am so happy to have signed for Arsenal. The Club has great history and a squad which is full of high quality players. This is a very exciting time for me and I'm looking forward to a successful career here." Diaby will take the number 2 shirt so he's obviously considered to be a first string player. It's going to be very interesting to see how quickly he is introduced to the squad, particularly as his position is one where we do actually have enough players. Is he ready to replace one of Gilberto, Flamini, Cesc? Whether he is or not, it's probably not the best news for the likes of Alex Song.

Diaby is joining up with his new team mates "immediately", so Auxerre's hope that they'd be able to hang onto him for the rest of the season looks like it won't be realised. Good.

The official site also has a factfile about him. One thing to note is that while his full name is Vasiriki Abou Diaby, he seems to answer to the name Abou (that's how Wenger referred to him).

He was captain of the French U19s who beat England 3-1 in the final of the UEFA championship in Belfast in the summer, and was raved about by everyone who saw him.

But possibly of greatest immediate significance to Arsenal fans is the fact that he has played for Auxerre in the UEFA cup this season, and hence won't be available for the Champs League (but see correction below!).

Rupe adds...
Actually, it's been suggested that players are only cup-tied if they play in the UEFA Cup from the group stage onwards, and Diaby only played in (both legs of) the qualifier against Levski Sofia. I'm having some trouble opening the UEFA regulations file (from here). If anyone can confirm what regulation 15.13 says, please do.

Update. Ok, got there in the end....

Having managed to find some software that'll convert the regularions pdf into a million jpegs (what a bloody hassle - handy program, though) I can confirm that it does indeed look like we can register Diaby for the Champs League.

Rule 15.12 is the one saying a club can register 3 new players so long as you do it before February 1st, and Rule 15.13 says that one of those players can have already played for their old club in a UEFA competition this season, so long as it's a different UEFA competition to that which the new club is playing in. Thanks go to Dave McKenna for his help in this.

Rupe adds...
And finally...

A lovely quote from the newswires...

Apparently Gerard Bourgoin, the Auxerre vice-president, met officials from both Arsenal and Chelsea to discuss Diaby....

"The choice belonged to Diaby, but Wenger was better than Mourinho, that's all".

All together now, "We all agree..."