Hmm, well it's one of those questions that is.. not the right question. All elements are positioned, the real seperation is between static (default) & relative positioning, in which elements must respect each-other in the element flow, and absolute & fixed positioning, in which they are removed from the element flow. All elements are following one positioning rule or another, so floating elements are positioned as well. However, you can't float absolutely positioned elements (I'm thinking fixed elements too, I've never checked).
It is like compairing a nail and a screw in a lot of cases, when trying to decide on how to position, or if you should float, since it depends on the job at hand more than anything. A good page will respect the element flow, don't abuse absolute positioning as the lazy option. If something seems like it should be part of the element cascade, then it probably should be! I dare say most elements should be, with the odd exception. If you take your time to fit the puzzle of elements together nicely in relation to each-other you will end up with efficient code, your pages will be less likely to fall appart, and of course it makes dynamic layout a lot simpler. Floating elements is still respecting the flow.