I wanted to give an update to this answer in case someone comes here looking for a way to do this as I did.
You can give the label a class with this option :item_wrapper_class => 'class_goes_here'
Here is a full example:
= user.input :resident,
:collection => [["In the U.S", true],["Outside the U.S.", false]],
:label_method => :first,
:value_method => :last,
:as => :radio_buttons,
:label => "Where is your principle residence?",
:item_wrapper_class => 'inline'
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…