CS5 bug with anchored objects on spreads
There's a bug in InDesign CS5 that you should be aware of, that makes anchored objects fail to print or output to PDF in certain cases.
The bug occurs if you have a facing pages document that contains text flowing from the left page of a spread to a right page. If you anchor an object to a text location on the right-hand page, but position the object on the left hand page, the object won't print or appear in a PDF exported as a print PDF. If you print or export to PDF with the "Spreads" option selected, the object does appear correctly. I'm running ID CS5 7.0.x on Mac OS X 10.6.6.
I tested this with the same files in InDesign CS4, and it appears to work correctly in that version.
Why would you ever need to create anchored objects that appear on adjacent spreads? Well, it might happen accidentally during the course of production, or it can be done intentionally with the techniques demonstrated here and here.
Remember, you can keep track of where your objects are anchored by choosing Type > Show Hidden Characters, and View > Extras > Show Text Threads, as I've done in the screen shot below.