In 10.6.5 I've noticed this irritating change (bug?). If a folder is selected in the navigation window -- even if it's not the focus, that is, even when the focus is the Save As: field, the Save button (and return) opens that folder. The only way to save to the current folder (shown in the pull down menu) is to deselect the shaded folder.
I'm convinced this is a bug, but a quick workaround is Command+Up Arrow then Commmand+Down Arrow. When you return to the original folder, nothing will be selected, and the Save button works as expected.
[crarko adds: I haven't noticed this behavior; can someone confirm this? And is it unique to 10.6.5?]

