The Landsat 7 L1T raster product should be supported in 10.1 but I pulled down a few samples from Glovis and reproduced the behavior as well. Additionally, I was unsuccessful in adding it to a mosaic dataset using the Landsat 7 ETM+ raster type, which should also be supported. It appears that 10.1 is not recognizing the *_MTL.txt metadata file required to work with the Landsat imagery as a raster product.
I wasn't able to find anything documented but I am leaning towards a possible defect or known (but undocumented) limitation. I am curious and plan to look further into the issue when time allows and will pass along anything I find out.