This appears to be a tool validation issue. If I create this same model in Model Builder, and run the "Make Mosaic Layer" portion of it, I can then validate the entire model and it will run OK - whether within Model Builder or subsequently via the tool's UI (provided default parameters exist and I don't change them). Whenever I do change the parameters, however, it errors with the message that %MosaicLayer%\Footprint does not exist.
I'd say that the tool is treating %MosaicLayer%\Footprint as a necessary prerequisite to running the model. If it's there (in memory?) due to previous runs of the model in Model Builder, then all's well. Otherwise it won't validate.
Looks like a bug? I am also running 10.1 (SP1).
However there is a workaround - using the Calculate Value tool in Model Builder. The model shown in the attached screenshot will run OK.