Probably you have already noticed that, but in Rails 3 got some new stuff going on. Same goes when testing Rails applications. Using errors.in(:key) is now deprecated so you'll probably need to change a whole bunch of tests and assertions. However, there is a quite fast way to do it. Open project in Netbeans and use Replace in project.
Go to Edit --> Replace in projects and type into corresponding fields:
Containing text: errors.on\((.+)\)\, Replace With: errors\[$1\].first\,
Select "regular expression" and press find. Wait few seconds for the results and press replace.
After that rerun your tests:
rake test:all # or if rspec rake spec