The dilemma faced by the average individual to set aside mobile devices while dining out becomes harder and harder the more we depend on the immediateness of these devices. We live in a "now" focused world where an incoming call is weighted with similar importance to the people we are sitting with.
There are two issues at hand here: The first, and generally more obtrusive one, is talking on a cell phone. The second, which more directly effects those at the table with you (and perhaps your server) is the text/BBM/mobile browsing/tweeting/etc. the silent, yet irritating distraction that halts conversation at the table.
Let's begin by addressing those who choose to talk on their phones in dining rooms where other people are enjoying a nice dinner. To be fair, this does not apply to every restaurant at every time of day. Someone on a cell phone in a restaurant that caters to business clientele at lunch time is not so heinous as a loud-talker in the middle of an intimate dining room in the evening.
The duration and volume of the conversation are almost as important as where you are having it. A quick "Hello? We're already here, see you shortly!" is okay, but discussing the finer details of "the thickness of conductor insulation required for minimum bending radius as a multiple of the cable diameter for units without metallic shielding" should perhaps be done whilst not having dinner, or at least be courteous enough to the other guests to head out into the foyer, or outside if the weather permits. There is little that is worse than having an intimate dinner ruined by someone on a cell phone loudly recounting his/her sexual adventures from the night before.
At the very least, be aware of volume! If you must have a conversation in the dining area, please have it in hushed tones - no louder than the general hubbub of the restaurant.
The second issue has more to do with being polite to your own guests and possibly being distracted when your server is trying to take orders etc. There is nothing wrong with 'checking in' on 4square or sending a quick 'tweet' about something delicious you just had/or are about to eat. These are great tools that you can use to promote your favourite restaurants and a large number of places offer WIFI with their guests in mind. Just make sure you're not distracted by an in-depth text conversation while your friend excitedly tells you their exciting news or while the server 'patiently' waits for your order.
As always, be courteous of those around you! Hopefully they will return the favour and everyone can enjoy their outing without disturbance.