Did you ever solve this problem? I'm having the same exact issue.
Found this tidbit at http://www.barneyb.com/barneyblog/2007/06/22/listcombobox-row-backgrounds-in-flex/
"For some reason, List will only call drawRowBackground if the 'alternatingItemColors' style is set for the List. So if you want to color your List rows, you have to supply that style, which you'll likely immediately be overriding."
Worked like a charm.