I created four ebuilds:

misc-misc/x-1.0.ebuild:
 DESCRIPTION="x blocks y: testing portage" 
 HOMEPAGE="http://www.example.org/"

 LICENSE="AS-IS"
 KEYWORDS="~alpha ~amd64 ~ia64 ~ppc ~ppc64 ~sparc ~x86"

 DEPEND="!misc-misc/y"

misc-misc/x-2.0.ebuild:
 DESCRIPTION="x blocks y: testing portage" 
 HOMEPAGE="http://www.example.org/"

 LICENSE="AS-IS"
 KEYWORDS="~alpha ~amd64 ~ia64 ~ppc ~ppc64 ~sparc ~x86"

 DEPEND="=misc-misc/y-2.0"

misc-misc/y-1.0.ebuild:
 DESCRIPTION="y blocks x: testing portage" 
 HOMEPAGE="http://www.example.org/"

 LICENSE="AS-IS"
 KEYWORDS="~alpha ~amd64 ~ia64 ~ppc ~ppc64 ~sparc ~x86"

 DEPEND="!misc-misc/x"

misc-misc/y-2.0.ebuild:
 DESCRIPTION="y blocks x: testing portage" 
 HOMEPAGE="http://www.example.org/"

 LICENSE="AS-IS"
 KEYWORDS="~alpha ~amd64 ~ia64 ~ppc ~ppc64 ~sparc ~x86"

 # Not good
 DEPEND="!<misc-misc/y-2.0 misc-misc/y"
 
 # OK
 DEPEND=">=misc-misc/y-2.0"