In LR 3 v 3.4.1, I use the "Enable Profile Correction" on my virtual copy and then select my camera and lens etc... afterward when I export the VR copy into a DNG the "Enable Profile Correction" does not get exported into the DNG file, so when I choose that option again on DNG, it make my picture a tad smaller frame again.
Ok here are the steps I took, using a Nikon D90 took a picture in High JPG format, imported it into my PC first as JPG (I used Picasa), than imported it from my PC to LR3 via Import wizard as JPG (use Add option), create a virtual copy of it, select the VR copy, switch to developer mode, went through and changed WB and few other settings, scrolled down to "Lens Correction", tick mark "Enable Profile Corrections" (in my case LR selected the correct lens and settings for me). Now go to menu File / Export / Export to DNG, now save it on my PC somewhere and turn around and Import the DNG into my LR catalog, and the "Enable Profile Corrections" is not set on the DNG copy of the picture. I just tried it again and I got the same results. Remark: It works if I start with a DNG formated picture.
I can sort of replicate it. If I export and import a rendered DNG file (as opposed to a raw DNG file), the Enable Profile Corrections is still checked and the sliders below are shown at the correct (adjusted) values, however the lens profile is no longer selected. It appears that LR's getting confused about whether it's a raw or rendered file.