Rack::Lint::LintError: Status must be >=100 seen as integer
Try deleting the SQLite DB - there seems to be a bug in data_mapper with changing the data structure and using old data. For me the bug went away after deleting the db and setting up a new one.
What version of ruby are you on because if you are on less than 1.9 you have to use the => hash constructor not : and move the colon to the beginning because it is a symbol.
User.create :username => "JoeSchmo", :email => "joe@schmo.com"
I had the same problem with Sinatra and datamapper. Creating my records with "new" keyword rather than "create" and then adding attributes one by one worked for me. Hope you find it useful.