I'm not saying Apple isn't worth a $1T market cap. I'm just saying you wouldn't use market cap to calculate if a company were a sound long-term investment. You'd look at the fundamentals, then make a determination of what you think the long-term value of the business is, then you could look at the current market cap and determine if it is under- or overvalued. Breathless reporting on the market cap of companies is sloppy, I think. The market cap kinda floats around, and worst case may be completely unrelated to the true long term value of a company.

Warren Buffet for some time said that he thought Apple was undervalued. I have no idea what he currently thinks, but it is probably still a reasonable value.