This is a quick & dirty hack to translate your book data from the CSV file produced by aNobii to the format accepted by GoodReads (and LibraryThing, apparently). The main advantages of this approach are that you will maintain your reviews and rating, and if GoodReads fails to find a book it will tell you the title (whereas if you just use a list of ISBNs, it won't).
Cons: I couldn't be bothered to mess with strptime, so you might lose your reading date; also, all your books will be imported with the default status ("read" for me).
You can download the script from here, it works with Python 2.4 and 2.5 (probably 2.3 as well). Feel free to improve it, I scratched my itch so I'm happy as it is.
UPDATE 2012-04-12: Alper Çugun updated and improved the script; if you plan to actually use it, please get his version from his Github repo.