The <%= is correct for rails 3 and <% is incorrect. With a <% in rails 3, the form won't display. I don't know why you would be getting a syntax error though.
Do I understand correctly that the situation is reversed in Rails 3
(i.e. <% is right for Rails 2.3, <%= is right for Rails 3)?
I'm using Rails 2.3. The Guide doesn't appear to refer to any
specific version. It was pretty confusing pasting something straight
out of the Guide and getting a somewhat opaque error message. I'd
hope to save someone else the trouble by at least marking up in the
Guide that this syntax has changed.