There aren't really a lot more details than what was teased up top. Arsenal need cover for the departing Gervinho and Marouane Chamakh - the normally-biennial African Cup of Nations is switching to an off-European Championship-year schedule, so there's another one this January, only a year after the last one. Henry's season with the Red Bulls just ended (the MLS transfer/loan window opens in mid-January), he's been back training with Arsenal, and Wenger said:
Will I sign him again in January? I don't know. I don't rule it out. I don't know if he would be interested. He is sharp.
I don't think there's an Arsenal fan anywhere who would be unhappy about this. I was somewhat skeptical when Arsenal brought him back last year - I lived through 2009 and 2010 Ken Griffey Jr., so I have some experience with legends coming back and ruining their reputation - but Henry's performance changed my mind right quick, and I think the way Arsene used him was about perfect. I would expect more of the same this time around, if it happens - the routine joy of seeing Henry in his proper colors interspersed with a few moments of absolute Thierry magic.
He won't singlehandedly vault Arsenal to the top of the table and to the Champions League final, but it's more Thierry at Arsenal, so I'll happily take it. Make it happen, Arsene.