I thought I might share some of the web sites I regularly visit for news and insight on the series. I'll add some more later when I reach specific topics of interest:
The official Agatha Christie website - obviously the one place to start if you want information on Agatha Christie and her stories. But they also have a reasonably active Community and a good 'News and insight' section, including so-called Christie papers (which look into different aspects of her stories and the adaptations) and the 'blog' of Mathew Pritchard, Agatha Christie's grandson.
The IMdB page - quite an active forum, where people discuss different aspects of the series. If you are looking for reviews of the episodes, this is also a good place to start.
David Suchet's Twitter - yes, the leading man is surprisingly active on Twitter. Some of the news he has posted so far: Filming of the final series will start in mid-October, script development meetings are taking place, Hugh Fraser (Hastings) is very likely to return, Pauline Moran (Miss Lemon) is more uncertain (and if you ask me, much as I love her character, that's a good thing, as anyone who have read my thoughts in the Poirot chronology would know!). So this is certainly a place to check once in a while if you want up-to-date information.
David Suchet - Fansite - Danish David Suchet fan Sanna is the mastermind behind this website. Full of information on Suchet, including lots of interviews and stuff on Agatha Christie's Poirot. She is usually very quick to post important news on the series.
David Suchet - Unofficial site - This website, created by Russian fan Daria Pichugina is absoluitely amazing! Loads of interviews and up-to-date info on both the series and Suchet himself.
ITV Press Centre - the place to go for news on the new episodes, including principal photography (when they start filming).
Mammoth Screen - the current production company of the series (and a lot of other UK crime series). Their Twitter is also a good place to stop by every once in a while (if you're interested in news of the production).
UPDATE: I just have to add one more - since I never seem to get around to adding a separate post on reviews and such: The Agatha Christie Reader. This is a jaw-droppingly fantastic site with insightful and detailed reviews of Agatha's stories, as well as almost every adaptation ever made of her books (including, of course, all the Poirot adaptations).