My notebook is the only machine I have with integrated graphics, yet I don't know of a way to force it to use integrated only. I also don't have a display with that resolution to test it, so I couldn't fully replicate your situation. I'll have a look at it later. There are a couple gpu settings that can be disabled if it's lagging without totally disabling OpenGL drawing. Last year the 2011 discrete mini wasn't really on my suggested list as I didn't think the gpu would be well supported in CS6 due to on board memory. When it was in beta, they suggested a requirement of 512MB of vram for OpenGL drawing. In the final release, they made the minimum requirement 256 with some functions unavailable. Those ones wouldn't work on the HD 3000 anyway. This is still a weird issue to me. It really shouldn't be laggy. It may be a driver thing or something. It really is too bad as that's the primary downside. A 640M + quad cpu at the $800-900 mark (which isn't unreasonable, especially as they don't even include the damn keyboard) would have made it a solid machine.
Bleh also just noted you had it set to basic. I need to research this further, so I can include proper reference if anyone else inquires about the mini. I really think it needs to be a solid machine though. It's a light desktop with basically nothing included. Considering the total cost configured, it should be a really good entry level solution. The facebook and email crowd are better served by an Air. Most of those guys don't really keep a dedicated space for a machine.
I think most of my issues are down to the size of my screen. (Though using very large brushes on adjustment layers is probably more related to the CPU.)
I should also emphasise that whilst Photoshop CS6 is laggy on my 2011 mini, it is for the most part very usable.
I'm working on large, multilayered 16 bit files (Nikon D700, D800, D3X, and large format film scans), so that's going to slow things down somewhat (compared to a user who is using CS6 for web design).
Also I regard CS6 as a work-in-progress and hope and expect that Adobe will issue updates that will speed things up.