It's not the problem here but don't use position:relative and floats together - causes bugs.
your problem is because of the box model hack. IE5.0 ignores any rule immediately following the hack, so you have to add some unecessary rule there to avoid losing your necessary rules.
some people simply add something like #a{ } after each use of the hack, this will allow IE5.0 to "recover" and the following declaration will be read correctly.
It's better all round though to add the "be nice to Opera" rule afterwards instead. Details of that are on Tantek's explanation of the BMH.
That's fuckin' ingenious, if I understand it correctly. It's a Swiss fuckin' watch.