I bet you're right, given previous casting decisions. Judi Dench would probably do an excellent job as Umbridge. I think Helen Mirren is a wonderful actress, but I can't see her in that role. Umbridge needs to be played by someone who is short and pudgy. Maybe Imelda Staunton or Vanessa Redgrave?
IMHO, Dame Judi is too old for Umbridge. I think Imelda Staunton (whom I adore) would be GREAT for the part.
In all honesty, I think the production team has made some wonderful casting decisions. I know some people who didn't like Kenneth Branagh as Lockhart (which reminds me -- I wonder if he'll come back for the bit part in the St. Mungo's scene in Order of the Phoenix), but I thought he was great as a smarmy liar. I know some people who didn't like the idea of Gary Oldman as Sirius, and I confess to having been one of them. But once I saw him, I realized they did the right thing. So, now I just let them do what they've done all along -- surprise and then impress me.
I've just finished looking at IMDb's cast list for Goblet of Fire, and it looks interesting with Miranda Richardson as Rita Skeeter and Ralph Fiennes as Volders. I'd read someplace that Kate Winslet was considered for Fleur, but I am so glad that person came to his/her senses. I've liked her in a lot of roles, but she's completely wrong for this one IMO.
And, the best news from IMDb is that GoF is scheduled for release here in the States on November 18. I know I'll be there!!!