Because Apple's stock is down, financial news outlets are on the prowl for an explanation. There is no good explanation, but the explanation that seems to have stuck is the risk of carriers cutting subsidies.
Carriers might cut subsidies, but Apple is not at risk. Let's assume that carriers worldwide cut subsidies across the board by $30. This chart from IDC shows smartphone market share in the most recent quarter:
We see Samsung with 29%, Apple with 24%, and everyone else combined with 47% of the market. Compare these market share numbers to these charts from Asymco showing the profit share of the major smartphone manufacturers:
What this shows us is that except for Apple and Samsung, smartphone manufacturers are either not profitable or marginally so. If every manufacturer made $30 less per smartphone, Apple would barely notice and Samsung would survive, but 47% of manufacturers would be wiped out. Apple will show lower profits for perhaps a year, but it should more than make up for lower margins with higher market share, increasing net profit.
If instead of reducing subsidies, the carriers charge an "upgrade fee" to subscribers as Verizon has done, unit sales will be deferred. The short-term effect will be reduced market size, causing gross margins to remain unchanged, but profit margins to decline as operating leverage takes its toll. This will again affect the weaker players disproportionately. As weaker players get taken out, Apple will take market share and maintain margins as subscribers pay the difference.
Either way, reduced subsidies are a threat to every smartphone manufacturer except Apple.
Furthermore, Apple stands to benefit more from the possibility of increased subsidization than any other market player. In the last 6 months, Apple sold 72 million iPhones, 27 million iPads, and 6.5 million laptops. iPads are 4G capable and not subsidized, and Apple was recently awarded a patent for a 4G laptop antenna and other manufacturers are making 3G/4G laptops. It stands to reason that MacBooks will eventually be 3G/4G compatible. Why wouldn't carriers subsidize 4G iPads and laptops if they drive incremental data plan sales? Verizon tried it when netbooks were popular, and Sprint is doing it now. Could it work with an Apple device? What would unit sales growth look like if these devices were subsidized?
Apple is down because that's what stocks do. Stock price movements don't always reflect the fundamentals. As far as subsidies are concerned the opportunities far outweigh the risks.