Stephen Fry just played Lady Bracknell in The Importance of Being Earnest.
As someone mentioned upthread, David Suchet played the role many years ago. This is nothing new
The reverse isn't seen where male roles are being converted for female performers - or at least I can't think of any off the top of my head. Happy to stand corrected.
Very much off the top of my head and I know there are definitely more:
Cynthia Evro as Jesus is JCS and as every (mostly male) part in her one woman Dracula
Judi Dench as Old Deauteronmy in that awful Cats movie
Pi in the Life of Pi stage show had both a male and female playing the role
I think I've seen that Spongebob has a female understudy in The Spongebob Musical who has performed the role
The Da Vinci Code play had some of the minor, but significant, characters played by both men and women (so it would be the grandmother or the grandfather depending for example)
At various times the lead roles in Shakespeare plays have been played by women
The Play That Goes Wrong I believe has had a female "Trevor" role at times, but that might be me being confused (it certainly could be gender swapped)
I think the problem is more dominant in panto lineups - you only need to look at the Crossroads Panto's across the country, Palladium included, to see they have their regulars and they're nearly all male. Even with female named character panto's (Snow White, Goldilocks etc) the female part is often relegated to the third row on the poster, beneath the male stars.
Lesley Joseph, Su Pollard, Linda Lucardi, Joan Collins, Anita Harris, Katie Price, Alison Hammond are all fairly well known on the Panto circuits. Look beyond Crossroads, Imagine are another big name plus places who produce in house or smaller companies. The named character in any panto is usually relegated to a smaller role (Dick Whittington, Aladdin, Jack too) because the really "good" roles are the villains, the fairies and the comedy roles and those go to the big names.