Unless an explicit height is set on relatively-positioned #wrapper, Opera treats its height as 0 and consequently places absolutely-positioned #foo (nested with #wrapper) at the very top of #wrapper for any percentage value of top, and at the very bottom for any percentage value for bottom (nice one Opera, how do you manage that if #wrapper doesn't have any height?)
Just in case I'm going mad, #baz is included to show that absolute psoitioning does still work outside of the relatively positioned #wrapper.
foo
foo
bar
bar
bar
bar
bar
bar
bar
bar
bar