What’s the iSlate worth? It’s not an easy question to answer, as Apple isn’t even confirming it’s got a tablet computer gadget in the works. But the market gives a rough guide to what Wall Street expects from the new device. Investors seem to be slapping an “iSlate premium” of some $25 billion on Apple’s value. Though Apple boss Steve Jobs’ skill at launching new products is unparalleled, meeting these hopes will be a tall order.