I added a simple back-annotation script to the wiki and thought I'd mention it.
http://www.alterawiki.com/wiki/back_annotation_script Note that I am not recommending back-annotating designs with this. As the link explains, node names change during synthesis, and so if you back-annotate your whole design, make a small change and resynthesize, many of the location constraints won't apply. Incremental Compilation with Design Partitions is designed to get around this limitation.
What I find this most useful for is locking down something specific, like a PLL or clkctrl block. (I may want to change the PLL or clkctrl location, but I use this to get the node name and syntax correct). It's also useful if your pins are floating, to maybe lock some down and then hand-edit the .qsf if you want them in another location. Just little things like that are the most useful.
