I actually wouldn't expect that to work unless you also mix in "div.span-x" or ".column" as these are the blueprint classes that have the required float i.e. (from blueprint screen.css):

.column, div.span-1, div.span-2, div.span-3, div.span-4, div.span-5, div.span-6, div.span-7, div.span-8, div.span-9, div.span-10, div.span-11, div.span-12, div.span-13, div.span-14, div.span-15, div.span-16, div.span-17, div.span-18, div.span-19, div.span-20, div.span-21, div.span-22, div.span-23, div.span-24 {float:left;margin-right:10px;}

We haven't quite got comma separated mixins working.

.prepend-8;
.span-6;
.column;
.last;

WIll work...

#header {
#title {
.span-10;
.column;
}

#logindisplay {
.prepend-8;
.span-6;
.column;
.last;
text-align: right;
}

#menucontainer {
.span-24;
.last;
text-align: right;
}
}