iOS Universal app - common pitfalls

A complete list of pros and cons? No, I don't think there is one.

But there are many sources of data where this can be gotten from, although I wouldn't call them "official sources". I had the same dilemma and this is what I found:

PROS

• From a maintenance and support perspective, one code base is easier to manage.
• A single release - updates and patches - covers all devices at the same time.
• Apple encourages universal apps, so the chance of getting featured on their "Featured List" is bigger. (I don't know this to be fact and I personally haven't heard from Apple on this...)
• Customer satisfaction is increased, because customers don't have to deal with or buy multiple apps.
• The increase in customer satisfaction could result in better reviews, translating into higher sales.
• Works on all Apple iOS Devices.

CONS

• Large memory usage. The largere graphics (and possibly higher resolution) that you will use for your iPad, in addition to those you use for the iPhone will make you app pretty chunky, and maybe even surpass the download limit for some service providers.
• You cannot set individual prices for the iPhone and iPad version, since you're just releasing one app with a single price tag.
• You can't see how your sales are split between the iPhone and iPad version.
• You are missing out on earnings generated by people willing to buy both the iPhone version and the iPad version of the app.
• With separate apps, you potentially launch your app twice - once per platform. This gives you two windows of great exposure (one for each platform) as opposed to one (for both platforms).

As can be seen, most of these Pros & Cons are related to sales and App Store release and whatnot, so if your app is free, or private (only distributed Ad Hoc) then a lot of these points won't really apply.

As you mentioned messaging and phone calls, you would have to use your own judgment on that depending on what your app will be used for, but iPhone or iPad or Universal does not have an affect on those. Those problems are device specific, and should be made compatible for both 3G and non 3G devices, as you can have both options for either device.

The most combined source for pros and cons I have found is here: On Going Universal.

In summary it boils down as to what your app will be for and what you want, but using the pros and cons above should help you come to a conclusion.