I don't know about the things not on the registry...so far, my Target registry experience is going well. They told me that anything could be returned for any reason (for exchange or regular gift card that can be used on any purchase).
That sucks that they didn't even read the message properly, Kel. Booo!
This reminds me of an idea I had a week or so ago...we have the book exchange, but since we are all going to have tons of babies soon, perhaps it might be useful to have an 'trade uneeded baby stuff' thread? Like if you have six unopened packages of newborn diapers and your bean was born at 10lbs.

I don't know how often this would be useful...I'm still running scenarios in my head...
I think the registry stays active for two months after the event.