In #53770 (and similar questions), I and a few others were unable to switch to a HiDPI mode in Lion, even on a Mac and Mac OS X that otherwise seem capable of supporting it.
For anyone who was in the same boat with Lion: did upgrading to Mountain Lion fix this problem for you?
(The release notes for ML don't mention anything I really need, so I was going to put off upgrading until at least 10.8.1, but if it allowed me to use HiDPI mode here, that would definitely be worth it today.)