Dementieva is playing the best she ever has. Safina is at the peak of her career. Svetlana just won a grandslam, so she is having one of her best seasons ever.
Azarenka/Caroline/Sabine/ are a constant threat, having a considerable amount of top 10 wins in the past 12 months.
I would say, its very competitive right now.
The only two players who could make a major difference are Maria and Ana. But you cannot have every top player playing their best at a time. (Momo hasnt played well since early 2007, and her 2009 is much much better than her past 2 years)
Apart from maybe Dementieva , Jankovic and Ivanovic are the two players who can challenge the WS in terms of speed, power and athleticism and their 2008 form seems to have deserted them. Svetlana is a threat only on clay and is the most inconsistent of the top 10 players , winning a GS one week then getting bagelled by Wozniak on grass the next.
Other than on clay, Safina will never be a threat to any Williams sister playing well. And even on clay she wouldn't have beaten Serena. Wozniacki isn't a threat right now.
The hard hitting youngsters coming up like Azarenka, Cibulkova and Lisicki will most likely be the ones who eventually topple them completely but it'll take 2-3 years at least for them to reach their peaks.
It's not that competitive right now , when you had Clisters (even though she had an abysmal H2H against either sister) , Henin , Mauresmo, Daveport , Hingis , Capriati e.t.c that was a competitive era , right now the WS are almost always the heavy favourites against any player.