I always have my Blackberry with me when I'm at the bookstore and I often use it to make notes or send an e-mail about a product. From time to time I've noticed a disapproving look from a store clerk, but I never thought much of it. It will be interesting to see if Amazon's newest service feature, TextBuyIt, causes brick-and-mortars to adopt new policies on cell phone usage.
Amazon TextBuyIt provides product search and purchase via text messaging. Once you select your item and make your purchase, Amazon then calls your mobile phone to confirm. The service isn't limited to books, of course, so you'll be able to use it when shopping for TV's, cameras and anything else Amazon sells. The service FAQs can be found here.
I've never seen any statistics but it's often been suggested that a large number of customers browse brick-and-mortar stores and then go home to make their purchase online. TextBuyIt only helps encourage this sort of behavior, so don't be surprised if you see a lot more in-store cell phone texting activity in the coming months.