In the final episodes of DS9, Martok became Chancellor of the Klingon High Council and Worf was named Federation Ambassador to the Klingon homeworld. However, in Nemesis, Worf is back in uniform and seamlessly resumes his position as tactical officer on board the Enterprise.
It's no surprise that he was present at the beginning of the movie as he was friends with both Riker and Troi so he would have good reason to attend their wedding.
I don't recall any instance of a Federation ambassador also being an active Starfleet officer. That said, I was surprised that Worf still had his commission and, apparently, being assigned to the Enterprise again in Nemesis after having been an ambassador for three or four years.
Also, following the events of the DS9 episode Change of Heart, when Worf was told in no uncertain terms that he was unlikely to get his own command, it would make more sense for him to pursue a career in diplomacy where there would be more opportunity for advancement.
So, is there any explanation for Worf being back in uniform, with his commission intact and no mention of him being or having been an ambassador in Nemesis?
I'm especially curious because, following the events of Nemesis with regards to the command staff of the Enterprise, there has been some speculation that Worf became Picard's new first officer (the deleted scene not withstanding) which seems odd because it seems like he would be going backwards in his career to take a terminal position.