I have an iPhone (and have had previous models) and wholeheartedly
recommend it as a smartphone for convenience and reliability. That
said, be aware of the total cost of ownership (phone plus expensive
plan for two years, or GSM-only contract-free phone plus cheap plan.)
A number of aviation-related apps are available, many of them very
cheaply. In-flight use isn't so great - no matter what software (and
XCSoar doesn't run), the battery drain is substantial when GPS is
running and data transfer takes place.

I also picked up a lightly used Dell Streak 5 Android PDA/tablet/phone
for use as glide computer. It cost me only US$200 and offers the best
screen among the portable, open-platform systems. It can easily be
powered and installing/updating XCSoar is a snap. At that price, I
don't see why one needs to compromise functionality. What Max
Kellermann says in this thread is true. (The Streak is too big for a
phone unless you carry a (man-)purse.)

Many Android smart phones will make a poor in-flight glide computer
due to their displays alone; do your research before you buy one.

PS.: there's an article in the current Soaring Magazine about PDAs,
but I find it fairly biased.