http://oboformat.googlecode.com/svn/trunk/doc/obo-syntax.html#8.4

basic summary - you are strongly advised NOT to use import directives in obo-format, they are not well supported. In cases where there is necessary (the only case of this should be dependence on obo-edit) then you should use OWL URIs and an oboedit.catalog file to map these. Stay tuned on the obo-edit-wg mailing list for details of oboedit.catalog support in OE.