There have been cries of mourning across the US over the past few months as the demise of unlimited mobile data plans has seemed imminent, but T-Mobile USA and Sprint have resurrected these plans amid consumer demand – paving the way for AT&T and Verizon to follow suit and shake up the US competitive landscape again, especially at the budget end. The challenge will be for Verizon and AT&T to decide how far they want to put their toes into the piranha pool of data price wars. In the past they have tended to focus mainly on higher value customers and on boosting profits, but as they come under pressure on all fronts from old and new rivals – such…