Now that Apple’s (NASDAQ:AAPL) iPhone is coming to Verizon (NYSE:VZ) next month, the brain-teaser for investors is trying to gauge what it means going forward for those two companies — and for formerly exclusive iPhone provider AT&T (NYSE:T).

In short, how many subscribers to Verizon’s wireless service will dump their current phones to snap up new iPhones, and how many of AT&T’s iPhone users will jump ship for a telecom provider whose only disadvantage to AT&T seemed to be that it didn’t sell an iPhone?

Thanks to a survey by ChangeWave Research, we have an indication, at least, of consumers’ initial intentions.

In a poll conducted of more than 4,000 people over the second half of December, 16% of AT&T subscribers said they planned to switch to Verizon if and when that company offered the iPhone. Sixty percent of AT&T customers said they wouldn’t switch, and 23% didn’t know.

Keep in mind, however, that this is all AT&T customers, regardless of whether they owned the iPhone. ChangeWave found that current Apple iPhone owners were the most likely group to jump to Verizon, with more than a quarter (26%) saying they’d leave AT&T.

And when will they make the move? For many, very soon. Among all AT&T customers planning to switch to Verizon, two in five (41%) say they’ll jump in the first three months — and another 31% in the first year.

The reasons behind AT&T customers making such a move are less surprising, and ChangeWave’s research has confirmed what have been the common complaints by consumers — in effect, that AT&T’s wireless service is simply less reliable than Verizon’s. When asked why they planned to switch, 42% of AT&T customers cited poor reception and coverage, while 27% pointed to dropped calls.

The irony of the dropped call mention is that, according to ChangeWave, AT&T customers have actually reported a decline in dropped calls since September, falling to 4.7% from 6%.

But brand loyalty — or lack thereof — can be a harder thing to budge. The survey found that AT&T led the pack among all wireless customers who planned to switch for any reason — 15% of AT&T subscribers said they were likely to jump within the next 90 days, compared with just 4% for Verizon.

Interestingly, that result for AT&T was a large jump from September, when only 10% of its customers planned on making a move. However, what seems more likely to have happened — a sudden increase in dissatisfaction, or the realization over the past month that Verizon was readying a place for which AT&T’s iPhone users to move?

Taken as a whole, ChangeWave’s survey suggests a bigger hit to AT&T from the iPhone’s move to Verizon than even the most dour Wall Street analysts have suggested. Analysts at equity research firm Hudson Square, for example, were among the more pessimistic, noting on Tuesday that they expected 4 million AT&T iPhone users to make the move to Verizon, or 17% of its iPhone customer base.

Considering that ChangeWave found that more than one-quarter of iPhone owners were definitely planning to jump, the numbers could be even worse for AT&T.

The other kernel in all of this, of course, is how phones running on Google’s (NASDAQ:GOOG) Android platform — up to now, a key part of Verizon’s strategy vs. AT&T and the iPhone — will fare now that Verizon will put its marketing muscle behind its partnership with Apple.