Does a computed column break 3NF (third normal form)?

Computed columns are not a construct of the relational model, so it doesn't make sense to try and evaluate them using relational model terms such as 3NF. Computed columns are a proprietary extension to SQL added by some vendors, and since they aren't logically stored (although you may persist them physically) they are not part of the tuple as they are not an attribute of the relation, but some expression based on it.

BTW - if that's the only modeling flaw you find in AdventureWorks, and most other sample databases for that matter, look closer :-)