I suppose you could, I never have personally. I don't think there is much chance with an apple device, I have been surfing for over a year on my iPad with no antivirus and have had no problems. I have an Android phone and use lookout which scans everything you download. I have never had a virus on any mobile device.
Blackberries are just as susceptible as other phones. I have an iPhone and even jailbroke it. Never had any issues. If the jailbreak starts acting up i just reinstall the apple software package
Viruses can get on Androids. There's a humorous one running around right now that pretends to be a pirated version of a legitimate app. Should you have the bad luck of installing this fake, it gives you a busted notice on your screen that makes an FBI arrest warrant look pleasant (not that I know from experience). In addition, it shoots off a text message to everyone on your contacts list saying something along the lines of this.
"Hey. I'm a stupid moron that would rather steal an app than pay $1 for it."