For aldiko/calibre novices:
To import your Calibre library books directly from Aldiko:
1) Go to "My Catalogs" on Aldiko, create a new catalog and give it a name.
2) Insert this http://youripaddress:8080/opds
in the URL dialog (your ip address should be something like 192.168.1.XX - you'll find it in your computer's network properties).
3) Turn on Calibre's content server, turn on nook's wifi, open Aldiko, tap on your newly created catalog and and...
4) ...Viola! Your entire calibre library arranged by author/title/tags etc.
5) Never transfer ebooks via usb ever again.
I posted these instructions because even though I've been lurking at MR since purchasing my nookcolor, (February of this year) and had read many MR members casually refer to it, it still took me a while to figure out how to set it up and I didn't fully appreciate how awesome it was until I did it.
Here's a couple of threads that helped: