as far as I can find, Netflix hasn't commented, even in secret, on whether they think comcast is throttling.
there is a report out there of a comcast subscriber who measured ~256kbps to netflix while using a vanilla setup, and ~3Mbps to netflix over a VPN running over comcast's network to an external peer point.
i don't believe verizon has implemented an on-demand video store that competes with netflix, but comcast has.
edit-->
here's the link to the report:
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forums.comcast.com/t5/Ba ··· /1961139comcast engineers try to explain it as different paths... which is of course true, but they don't explain why the network wouldn't balance out the usage naturally (using agnostic load balancing), unless they're shunting all netflix traffic to a particular over-burdened or under-provisioned edge node, which effectively throttles the singled-out service.