Well, it's about time. The magical presence that is Bill Nighy will apparently be gracing Harry Potter and the Deathly Hallows, according to a report in The Independent. The conjecture is that he'll play Minister of Magic Rufus Scrimgeour, on whom more below (spoiler-phobes, skip the following paragraph).
Scrimgeour succeeds Cornelius Fudge as Minister of Magic towards the end of The Half-Blood Prince and until he is killed in the course of the Deathly Hallows. Scrimgeour tries to undo Fudge's mistakes by reaching out to Potter and attempting to co-opt him into the Ministry's anti-Voldemort campaign, but is rebuffed by Harry, who's understandably sick to the teeth of the Ministry's inconsistent attitudes. He is later placed under the Imperius Curse by Death Eaters and tortured for word of Potter's whereabouts, being killed when he refuses to co-operate - so he gets a rather heroic death in the end.
This is still a rumour, but as one of the few British character actors who hasn't appeared in Potter yet, Nighy's surely a shoo-in *some *sort of role - and he would not be a bad fit in the small role of Scrimgeour, who's described as looking like an "old lion" with bushy eyebrows, tawny hair, yellow eyes and wire-rimmed spectacles.
Harry Potter and the Half-Blood Prince is out on July 15. Harry Potter and the Deathly Hallows will be released in two parts, in November 2010 and July 2011.