I've just released my GridFieldRelationHandler components at as module that you can get from [url=http://s.geek.nz/53]here[/url]. This module provides two components, one for a has_one and one for a has_many or many_many, that emulates the radio buttons/checkboxes from the HasOneCTF, HasManyCTF and ManyManyCTF from SilverStripe 2.4 in GridField. See the attached screen shot for an example with a has_many relationship.
Before SilverStripe 3.0.3 or the current 3.0 branch on github, this does need a patch to framework/forms/gridfield/GridFieldPaginator.php to get the has_many/many_many component working properly with pagination. That patch is available [url=https://github.com/silverstripe/sapphire/commit/d2b4e0df01f82fdbe613890c8ae909af404640a5]here[/url].
For more information, consult the [url=http://s.geek.nz/54]readme[/url].