background-position on UL with floated LI is not working test-case in URL
Everything is working fine. Try putting a border on the <ul> to see what's happening (there are no in-flow children, so height becomes very small, but the background is correctly placed at the bottom left of that).
Re-opening, quirks mode should consider that the floating children extend the height http://jscript.dk/2002/8/moz/floatbg.htm
No. Quirks mode should not consider that, because floats in fact do not expand the height. Quirks mode is about being compatible with common authoring practices that happen to work in NS4 and IE. This is not such a practice, by any means...