They are retail mongs, not many people working in chain stores actually know much about the products.
But not all.
There are good and bad ones. They cant be expected to know all of their products as in depth as to know how to turn on live view in an SLR.
I have owned 5 Canon SLR's 4 had live view and all 4 had different ways of turning it on, i doubt i would be able to turn on live view on a Nikon and i consider myself at least half knowledgeable in SLR's.
Rant over.
Buy the 550d. Its not that much more and by buying the best you wont be able to regret not buying the better one.