My answer to this question was, "never" until a couple of years ago. Sure, I was more likely to make a phone call on my cell phone than on my computer, but that is to be expected - it has traditionally been designed to be a voice communication device.
Opting for my cell phone rather than my laptop first started for me a couple of years ago. I began using SMS as a substitute for email. Then I started using email on my Blackberry because it would boot faster than my computer. Next came Google SMS - for me it was soooo much faster to get a phone number for a business through Google's SMS service than to call (ok, which costs money) or look online. Then, I got an iPhone and started downloading all kinds of applications. Some I barely use, but .... there are quite a few that I use rather than comparable experiences on the PC. These include Facebook (I'm more likely to be doing something interesting when I'm out and about), Scrabble (tallies the score for you), and maps (stopped printing all those maps out) among others.
For all of these services whereby I opt for my phone rather than my PC, I do so because the experience on the cell phone is more convenient. That means the benefits outweigh the inhibitors to use. When it comes to mobile services, there is convenience when there is value to the immediacy of the information or service, tasks are simple to execute and there is context - like my location.
We lay out this framework in our newly released report, "The Convenience Quotient of Mobile Services: A Facebook Case Study."