Clubs are certainly allowed to poach youngsters. One of the main planks of the Elite Player Performance Plan (EPPP), the document which lays out the changes to the Academy system, curently in their first year of imlementation, makes it much easier for clubs to do this than previously.
Pembo has VERY close links with senior figures at Forest's academy. It's rumoured that their academy, which already has a strong reputation on the circuit, is in line for substantial new investment from the new owners. They are also looking to recruit to new positions created by the need to meet the new EPPP directives.
Add it all up, and it doesn't look great for the Blades. Of course, there are plenty of other places Pembo could end up !