Does this mean you aren't testing yourself? If so, I've checked in standalone versions, and the alignment is almost exactly the same. The major difference is that 5.5-ie7 do not "shrink-wrap" floats, so (on the provided code) div#cheaper-promo expands the full width of the viewport.
(This is also because the float over-rides the display:inline and sets a default display:block. Combine that with not shrink-wrapping floats in older ie and the element expands full-width. )