Backspace and Alt-Left run different commands by default.
Alt-Left goes back. (Go BACK)
Backspace goes up. (Go UP)
'Back' and 'up' are the same folder if you just came from the parent but in other cases they are different folders.
Opus only remembers the folder selection if a folder is pulled from the history. The Go BACK UP command Steve mentions tells Opus to go up a level and, if possible, pull the folder from the history so that the file selections, scrollbar position, etc. are as they were when you were in that folder a moment before. On the other hand, running just Go UP will read in a brand new view of the folder with those things reset.
More about this in the FAQs:[ul][li]HOW TO: Remember scroll positions and file selections.[/li][/ul]