Analysts have been scratching their heads trying to figure out the reason for the iPhone shortages. Now a third analyst from RBC has weighed in: the reason is surging demand.

Previously, Piper Jaffray analyst Gene Munster thought that supplies were drying up because a 3G iPhone launch was to be earlier than anticipated. Yesterday, Bernstein Research Toni Sacconaghi thought it might be due to a parts shortage, also affecting Sony Ericsson.

However, Mike Abramsky with RBC thinks that several other factors have conspired to create the shortage, namely a huge post holiday demand that never died down. "Rather than from component shortages or ahead of a 3G launch, Apple may have under-estimated post-holiday demand on expectations of a [calendar] Q1 sales slowdown, which never materialized."

Mr. Abramsky noted that both Apple and investors have struggled to forecast demand. The recent weakening of the dollar, creating huge demand for unlocked phones overseas is also being discussed today in several circles, including our own forums at iPO/TMO .

Apple’s dilemma now is that if it ramps up production, it could have too many on hand in inventory when the 3G iPhone is announced. However, if it doesn’t, then sales expectations, not met, could cause the stock to take a hit. When Apple reported slowed iPod growth last quarter, their stock took a major hit, and Apple doesn’t want that to happen with the iPhone. In any case, each day’s analysis sheds more light on the topic, and AAPL goes up and down accordingly.

Yesterday, AAPL was down, perhaps because investors thought that a parts shortage might keep Apple from meeting demand. Today, AAPL is up, perhaps because investors see Apple selling iPhones overseas like hot cakes, driven by a favorable exchange rate, and unable to keep up with demand. The story continues to unfold.