Just registered for this thread. Actually, I'm SwitchresX' developer, so maybe I can just give direct reply here.
On the HD3000, it's something that has been added on 10.8.3 only, before that the HD3000 could only create scaled resolutions below the native one. Now it can go above.
I have a MBP2011 with a HD screen. When using SRX, I can create and enable a resolution like 3840x2400 (and a 1920x1200 HiDPi), but only with the Intel card. It is enabled, but I can't use it: the display goes gray, and I can only reboot.
When it switches to the Radeon, it loses this resolution also. That means that this resolution is too high for getting enabled on the AMD card also (and then also too high to be displayed).
There seem to be a limit when using the Intel card for enabling a new scaled resolution, and an other one for using it; and again an other limit when using the AMD card.
(BTW I previously told 'adnbek' that it doesn't work on AMD. Obviously I was false - I forget myself which video chips I do have on all the Macs I am testing my software on...)