NEW YORK ( ETF Digest) -- During the highly anticipated launch of the iPad 3, Apple ( AAPL) CEO Tim Cook pointed out that in the last quarter, its flagship iPad sold more units -- 15.4 million to HP's ( HPQ) 15.1 million -- than any other PC manufacturer sold PCs. It appears that the post-PC era is nearly upon us, and it's being ushered in by a truly transformational company. So now is probably as good a time as any to take a look at the effect this juggernaut is having not just in the world of technology, but on ETFs.Just recently Apple passed up Exxon Mobil ( XOM) to become the most valuable publicly held company in the world by market capitalization, at a staggering $504 billion (as of March 8). This astonishing public valuation has had some unexpected effects that investors should be aware of. Chief among them is the risk of overconcentration, as a great many indices and the ETFs that track them are weighted by market cap.