Text highlighting is a problem with absolutely positioned elements in IE/Windows. I've never heard of anything like that for floats. You might be thinking about the peekaboo bug which hides some text completely in IE6.
As Vinnie said its the text highlighting (selection) in absolutely placed elements that ie6 can't handle. You can trick ie6 into quirks mode using the xml prologue and then you will be able to select it but you will then have to work in quirks mode (which isn't a major problem).