Man United sold their most productive player (Ronaldo) as well as Tevez and got Valencia who isn't by any means near Ronaldo's level and the injury-prone Owen who past his best. So definitely United 09/10 are weaker than United 08/09/10.
Liverpool lost one of their most influential players Xabi Alonso and Sami Hyypiaand replaced them with Aquilani, who has zero experience at Premier League, and Kirgiakos who isn't at the same level as Hyypia. They got Glen Johnson who is more adopted to the English game than Arbeloa, but their bench is still short of players and Liverpool looks weaker this season.
Arsenal were forced to sell Adebayor and Toure ,two of their key players, to City and got only Vermaelen, who is a very solid defender indeed. The only advantage for them is that their team is one year older, hence more experienced, but the loss of Adebayor (way better than Bendtner) and Toure make them a weaker team.
Only Chelsea are building on their previous squad, they have kept all important players and brought Zhirkov, who could be a real bargain for them. The only problem for them is that their squad is one year older and every further year could be crucial for ageing players such as Drogba, Anelka, Ballack, Lampard etc.
On the other hand Inter, Real Madrid and Juve look like they have imroved their teams. So yes, there is a little decline for the English top clubs