I asked E*Trade about this some time ago. With their new direct-access CFD service, which I don't use, then your orders go immediately on the order book (I saw this on one of their live online demos, which incidentally is very entertaining and informative, well worth having a look at).
However with their standard quote-driven CFD service, it's not so simple. I was concerned that E*Trade might be taking the other side of my trades, especially as (to me anyway) their T&Cs weren't clear on this. I was categorically told that this doesn't happen, that they only act as broker, and that the orders are routed to CFD market makers. When I asked who the MMs for CFDs were, and how they operated relative to the equity market, all I got was 'that's confidential'. Hmm, curious.
In practice though I *always* get instantly filled at the quoted price with order sizes of £5-£10K so I'm not concerned, although I would like to find out how it really works.
KenN